Tap output collimator

ABSTRACT

A surface-emission laser diode includes a distributed Bragg reflector tuned to wavelength of 1.1 μm or longer, wherein the distributed Bragg reflector includes an alternate repetition of a low-refractive index layer and a high-refractive index layer, with a heterospike buffer layer having an intermediate refractive index interposed therebetween with a thickness in the range of 5-50 nm.

BACKGROUND OF THE INVENTION

[0001] This invention generally relates to laser diodes and further tothe art of optical telecommunication that uses a laser diode. Especiallythis invention is related to a so-called surface-emission laser diodethat emits a laser beam in a generally vertical direction to a substratesurface. Also, the present invention is related to an opticaltransmission/reception system and optical-fiber telecommunication systemthat uses such a surface-emission laser diode. Further, the presentinvention relates to a semiconductor distributed Bragg reflector andalso a surface-emission laser diode and further a surface-emission laserarray. Further, the present invention relates to a surface-emissionlaser module, an optical interconnection system and an opticaltelecommunication system.

[0002] A surface-emission laser diode is a laser diode that emits alaser beam in a generally vertical direction from a surface of asubstrate. By using surface-emission laser diodes, two-dimensional arrayintegration of laser diode is achieved easily. Further, the laser diodehas an advantageous feature of relatively narrow divergent angle of theoutput optical beam (about 10 degrees), which is particularly suitablefor coupling with optical fibers. Furthermore, inspection of the laserdiode device is made easily in a surface-emission laser diode.

[0003] Thus, surface-emission laser diodes are suited to construct anoptical transmission module (optical interconnection apparatus) ofparallel-transmission type, and research and development are conductedprosperously. The immediate application of the optical interconnectionapparatus would be the parallel connection between computers or circuitboards in a computer, including short-range optical-fibertelecommunication. In future, application to a large-scale computernetwork and trunk line system of long-range, large-capacitytelecommunication is expected.

[0004] Generally, a surface-emission laser diode includes an activelayer of a group III-V semiconductor material such as GaAs or GaInAs,and an optical resonator is formed by disposing an upper semiconductorBragg reflector and a lower semiconductor Bragg reflector arrangedrespectively above and below the active layer.

[0005] In such a construction, the length of the optical resonator isremarkably short as compared with the case of an edge-emission laserdiode. Therefore, it is necessary to increase the reflectance of thereflector to a high value (99% or more) for facilitating laseroscillation. Because of this, it is practiced to use a semiconductorBragg reflector in a surface-emission laser diode as a reflector,wherein a semiconductor Bragg reflector is formed of an alternate andrepetitive stacking of a high-refractive index material such as GaAs anda low refractive index material such as GaAs with a period of ¼wavelength.

[0006] However, in the conventional semiconductor Bragg reflector thathas the structure mentioned above, there arises a spike structure in theenergy band as a result of band discontinuity at the hetero interface,at which the materials of different bandgaps are jointed, and the spikestructure thus formed tends to function as a barrier against carriers.Thereby, there arises a problem in that the semiconductor multilayerpart increases the resistance of the laser diode. Because of this,conventional surface-emission laser diodes constructed on a GaAssubstrate have suffered from the problem of comparatively high operatingvoltage of about 2.5 volts or more. Because of this, it has beendifficult to use the surface-emission laser diode with a CMOS driverintegrated circuit, which produces a laser driving voltage of 2 volts atbest. The itemization of this operating voltage of 2.5 volts is: 1.5Vfor the diode part; and IV for the device resistance. In order to reducethe operational voltage below 2 V, it is necessary to reduce the deviceresistance by one-half, while it is extremely difficult to meet for thisrequirement at the present stage of technology.

[0007] In the case of a laser diode of long-wavelength band for use inoptical telecommunication, such as the laser diode of 1.3 μm band or1.55 μm, a low voltage operation is expected in view of the fact thatonly a voltage of 1 volt or less is applied to the diode part of thelaser diode. Unfortunately, the desired low voltage operation is notmaterialized in such a long wavelength laser diode. In conventionallong-wavelength laser diode, InP is used for the substrate and InGaAsPis used for the active layer. In such a system, the lattice constant ofInP constituting the substrate is large, and it is difficult to achievea large refractive-index difference in the reflector when a materialthat achieves lattice matching with the InP substrate is used for thereflector Consequently, it has been necessary to stack 40 or more pairsin the reflector for realizing sufficient reflectance. In such aconstruction, however, the resistance of the reflector increases againas a result of increased stacking number of the reflector. Thus, it hasbeen difficult to drive the laser diode driver by a CMOS integratedcircuit.

[0008] In a surface-emission laser diode formed on an InP substrate,there is another problem of change of laser characteristic caused by thetemperature. Because of this, it has been necessary to add an apparatusfor stabilizing the temperature in the laser diode constructed on suchan InP substrate. However, the use of such a temperature regulator isdifficult in the apparatus for home use, which is subjected to a severedemand of cost reduction. Because of these problems of increased numberof stacking in the reflector and the poor temperature characteristics,practical long-wavelength surface-emission laser diode has not yetcommercialized.

[0009] In order to deal with the foregoing problems, there is a proposalto construct a surface-emission laser diode on a GaAs substrate by usingan AlInP layer, which achieves a lattice matching with the GaAssubstrate, in at least one of the upper and lower semiconductor Braggreflectors as the low refractive index layer, and further by using aGaInNAs layer in at least one of the upper and lower semiconductor Braggreflectors, as disclosed in Japanese Laid-Open Patent Application9-237942, such that a large refractive index difference is realized inthe reflector and the number of stacking therein is reduced whilemaintaining high reflectance.

[0010] In the foregoing conventional art, the bandgap of the activelayer is reduced by 1.4 eV by using GaInNAs, in which N is introducedinto the III-group V semiconductor material system of GaInAs. As aresult, the laser diode can produce an optical beam with a wavelengthlonger than 0.85 μm. In the aforementioned prior art, it should be notedthat the material system of GaInNAs can achieve a lattice matching withthe GaAs substrate. Further, the prior art describes the semiconductorlayer of GaInNAs can be a promising material for the long-wavelengthsurface-emission laser diode operable in the 1.3 micron band and 1.55micron band.

[0011] In spite of such a description in the prior art with regard tothe possibility of surface emission laser diode operable in thewavelength band longer than 0.85 μm, there has no such a laser diodeactually materialized. The present situation would be something likethat the theoretical construction is already established but the actualconstruction for materializing the laser diode is not discovered yet.

[0012] In one example, there is a laser diode that uses a semiconductorBragg reflector formed by stacking high-refractive index material layersof GaAs and low refractive index material layers of AlAs alternately asnoted above with the periods of ¼ wavelength. However, the laser diodestructure thus formed does not provide optical emission at all, oroperates but only with low power, indicating that the efficiency ofoptical emission is extremely small.

[0013] Similarly, there is a laser diode disclosed in the Japanese LaidOpen Patent Application 9-237942 in which an AlInP layer is used for thelow refractive index layer of the semiconductor Bragg reflector. In thiscase, too, the luminous efficacy of the laser diode is far from thelevel of practical use.

[0014] The reason of this unsatisfactory result is attributed to thechemical activity of the material including Al. More specifically, it isthought that the use of a material containing Al easily invitesformation of crystal defects originating from Al. Thus, there have beenproposals, as in the Japanese Laid-Open Patent Application 8-340146 andJapanese Laid-Open Patent Application 7-307525, to construct thesemiconductor Bragg reflector with materials free from Al such as GaInNPand GaAs. However, the material system of GaInNP and GaAs can provide arefractive-index difference of about half as compared with the materialsystem of AlAs and GaAs. Thus, the stacking number in the reflector hasto be increased, and the object of reducing the resistance of thesurface-emission laser diode is not attained.

[0015] Thus, at present, the surface-emission laser diode operable atthe long-wavelength of 1.1-1.7 μm does not exist, and because of this,it is not possible to construct a computer network or optical-fibertelecommunication system that uses such a laser diode.

[0016] As explained before, in a conventional surface-emission laserdiode, it was also not possible to use a CMOS circuit for the laserdiode driver, and it has been necessary to use an expensive specialdriver circuit. On the other hand, if a mass-produced CMOS driverintegrated circuit could be used, the cost of the opticaltelecommunication system that uses such a surface-emission laser diodewould be reduced significantly.

[0017] Furthermore the use of a CMOS circuit can reduce the power supplyvoltage of the driver integrated circuit as well from 5V to 3.3V. Withthis, it is possible to reduce the power consumption of the system toabout one-half, and a very large effect of electric power saving isobtained.

[0018] As noted before, there is a widespread expectation ofoptical-fiber telecommunication in relation to computer networks, andthe like. Especially, there is a need of realizing a low cost system inorder that the public accepts such an optical telecommunication system.Unfortunately, the surface-emission laser diode that can be used forthis purpose and can be used with a low-cost CMOS driver integratedcircuit, and oscillates at the long-wavelength band of 1.1-1.7 μm doesnot exist. Hence, the telecommunication system that uses such a surfaceemission laser diode does not exist.

[0019] Meanwhile, in the above-mentioned semiconductor Bragg reflector,in which semiconductor layers of different bandgaps are grownalternately, there arises the problem of spike formation in the bandstructure thereof at the hetero interface as a result of the banddiscontinuity. When such a spike structure is formed, the spikestructure functions as a barrier with regard to the carriers. Thus,there arises a problem in that the electric resistance becomes very highin the semiconductor multilayer part of the surface-emission laserdiode. This effect also contributes to the large drive voltage of 2.5 Vfor the surface-emission laser diode constructed on a GaAs substrate. Asnoted previously, it has been difficult to drive a laser diode havingsuch a large driving voltage by the driver integrated circuit formed ofa CMOS circuit (driving voltage is below 2 volts).

[0020] As noted previously, the itemization of this operating voltage of2.5 volts is: 1.5V for the diode part; and 1V for the device resistance,and it is necessary to reduce the device resistance by one-half in orderto drive the laser diode with a drive voltage below 2 volts. However,this is a very difficult subject.

[0021] Recently, the optical systems are used also for peripheraltransmission/reception systems, and there is a widespread expectationabout the computer networks using the optical-fiber telecommunicationtechnology including such a peripheral transmission/reception system.Especially, there is a keen interest about a low cost optical systemrequired for spreading of the optical fiber technology to the generalpublic. However, the surface-emission laser diode that can be used forthis purpose and can be used with a low-cost CMOS driver integratedcircuit, and oscillates at the long-wavelength band of 1.1-1.7 μm doesnot exist yet. Hence, the telecommunication system that uses such asurface emission laser diode does not exist at the moment.

[0022] In such an optical-fiber telecommunication system that uses thelong-wavelength surface-emission laser diode operating at the wavelengthband of 1,1-1.7 μm, the photodetection device constructed on a Sisubstrate cannot be used, as such a photodetection device cannot detectthe wavelength of 1.1-1.7 μm. In such a system, it is necessary to use aphotodetection device that has a sensitivity to the wavelength of1.1-1.7 μm. However, the photodetection device that has sensitivity tothe desired wavelength band of 1.1-1.7 μm is expensive as compared withthe low cost Si photodetection device. Thus, simple replacement of aconventional Si photodetection device with the photodetection devicehaving the sensitivity to the wavelength of 1.1-1.7 μm causes anincrease of cost of the whole optical-fiber telecommunication system.Thus, in order to realize an optical telecommunication system that usesthe long-wavelength surface-emission laser diode of 1.1-1.7 μm band, anapproach other than replacing the conventional Si photodetection devicewith an expensive photodetection device is needed.

[0023] Furthermore, a GaInNAs active layer having a high strain is usedin the long-wavelength surface-emission laser diode, as will beexplained below. In such a laser diode, deterioration of devicecharacteristic may be caused as a result of the thermal stress caused bythe difference of linear thermal expansion coefficient with regard tothe mounting substrate.

[0024] Meanwhile, in the optical-fiber telecommunication system thatuses a surface-emission laser diode, it is possible to arrange a numberof laser diode elements, each formed of a surface-emission laser diode,with high integration density. Thus, the distance between the opticalfibers can be reduced as compared with the case in which a conventionaledge-emission laser diode is used for the laser diode array. Generally,optical fibers accommodated in an optical cable is provided with amarker band or a plastic ring in the form of a coloring layer oridentification code (ID mark), in order to allow identification of thetransmission line. When the distance between the optical fibers isreduced, the space available for these protection layers or rings isreduced.

[0025] In the production of an optical module that accommodates thereinan array of surface-emission laser diodes, it should be noted that theproduced optical module would becomes a defective product unless anecessary quality is secured for a predetermined number of laser diodeelements in the array. Otherwise, the product loses the value thereof.

[0026] This issue is related to the yield of the laser diode productionprocess. In the production of the module product that uses an arrayarrangement of the laser diode elements, there is an acute demand ofestablishing the production process in which the modules that functionnormally are utilized efficiently and the yield of production of themodule is improved.

[0027] Summarizing above, there is no available long-wavelengthsurface-emission laser diode operable at the wavelength band of 1.1-1.7μm and that there is no available optical transmission/reception systemthat uses such a laser diode.

[0028] Also, it is known in the art of surface-emission laser diode toprovide a structure in which a current confinement layer (Al₂O₃) in apart of the p-type semiconductor distributed Bragg reflector close tothe active layer by oxidizing an Al(Ga)As selective oxidation layer forthe purpose of reducing the threshold current density. It should benoted that the current confinement layer of Al₂O₃ is a good insulatorand the holes constituting the carriers are injected into a limitedregion of the active layer as a result of the action of the currentconfinement layer, and it becomes possible to increase the carrierdensity easily to a threshold carrier density needed for causing laseroscillation. Thereby, it becomes possible to suppress the thresholdcurrent to sub milliamperes. Because of the fact that the refractiveindex of this selective oxidation layer is smaller than the refractiveindex of the semiconductor layer, the selective oxidation layerfunctions as an effective optical confinement layer for confiningtransverse mode, and it becomes possible to obtain a fundamentaltransverse mode oscillation in the case of reducing the confinementdiameter to below about 4 μm in the case of the device is designed forthe 0.98 μm band.

[0029] In the device in which the confinement diameter is reduced toabout 4 μm or less as noted above, on the other hand, there arises aproblem of increased electrical resistance because of excessive decreaseof the current path area in the current confinement structure. In thedevice in which the confinement diameter is reduced to the above size orless, for example, it should be noted that the confinement resistancecaused as a result of such a current confinement structure constitutesmore than the half of the device resistance. As such increase ofresistance of the device can become the cause of various problems suchas increase of operational voltage, saturation of output power caused byheating, decrease of modulation speed, and the like, it is necessary toreduce the resistance of the confinement structure. This includes notonly the reduction of resistance of the current confinement regionitself but also the resistance of the peripheral part of the currentconfinement region.

[0030] With regard to the cause of such an increase of resistance as aresult of using a current confinement structure, it should be noted thatthere is a substantial contribution from the high resistance of thep-type semiconductor device used in the p-type semiconductor distributedBragg reflector. In a semiconductor material, there appears a very highpotential barrier at the hetero interface where two semiconductor layersof different bandgaps are contacted, and becomes of this, a p-typesemiconductor distributed Bragg reflector generally shows a very highresistance as compared with an n-type semiconductor distributed Braggreflector.

[0031] Conventionally, it is known in the art of surface-emission laserdiode of 0.98 μm wavelength to provide a heterospike buffer layerbetween the two layers having different Al contents and forming a p-typedistributed Bragg reflector, for reducing the electric resistance of thedistributed Bragg reflector, such that the heterospike buffer layer hasa composition intermediate of these two semiconductor layers ofdifferent kind. Reference should be made to Photonics TechnologyLetters, Vol.2, No.4, 1990, pp.234-236, Photonics Technology Letters,vol.4, No.12, 1992, pp.1325-1327.

[0032] Thus, in the art of surface-emission laser diode, decrease ofresistance of the device is an important subject matter, and activeresearch and development are being made especially with regard to thereduction of resistance of p-type semiconductor distributed Braggreflectors. For the desired reduction of the resistance, the use of thehetero barrier buffer layer noted above is extremely effective. Further,it is similarly very effective to increase the doping concentration ofthe semiconductor layers constituting the semiconductor distributedBragg reflector, especially the semiconductor layers including theheterospike buffer layer and the layers in the vicinity of the foregoingheterospike buffer layer.

[0033] In the case of using a highly doped p-type semiconductor, it istrue that the electric characteristics such as device resistance areimproved, while there also arise problems such as conspicuous freecarrier absorption caused by holes or conspicuous intra-valence bandabsorption. Thereby, the optical properties of the laser diode aredegraded. To improve the electric power transformation efficiency in asurface-emission laser diode, it is particularly important to reduce theabsorption of the laser beam by the p-type semiconductor distributedBragg reflector, while this requirement of reduction of opticalabsorption loss contradicts with the requirement of reduction ofelectric resistance.

[0034] To eliminate this problem, Japanese Laid-Open Patent Application2001-332812 proposes a surface-emission laser diode having asemiconductor distributed Bragg reflector in which the dopingconcentration of the semiconductor distributed Bragg reflector is maderelatively low for the region located at the side of the active layerwith respect to the region away from the active layer such that thebandgap difference between the two different semiconductor layers ofdifferent refractive indices and constituting the semiconductordistributed Bragg reflector is reduced.

[0035] In this conventional art, the doping concentration of thesemiconductor distributed Bragg reflector located in the vicinity of theactive layer is set lower than the doping concentration of otherregions, for minimizing the deterioration of the optical output causedby the influence of optical absorption by the semiconductor distributedBragg reflector. Further, in order to prevent the increase of electricresistance of the semiconductor Bragg reflector caused as a result ofreduced doping concentration, the difference of the bandgap is reducedfor the semiconductor layers constituting the foregoing less dopedregion of the semiconductor distributed Bragg reflector such that thepotential barrier height formed at the heterointerface is reduced. Inthe surface-emission laser element having such a construction, thesaturation point of the optical output is increased while simultaneouslyreducing the device resistance.

[0036] Thus, in Japanese Laid-Open Patent Application 2001-332812, thedoping concentration is reduced in the region located in the vicinity ofthe active layer in the purpose of reducing the optical absorption, andthe bandgap difference between the two semiconductor layers of differentkinds and constituting the semiconductor distributed Bragg reflector isreduced for preventing the increase of electric resistance.

[0037] However, such a construction, while being able to reduce theresistance to some extent by reducing the bandgap difference for thesemiconductor layers constituting the heterointerface, has stillsuffered from the problem that the electric resistance cannot be reducedsufficiently due to the fact that the reduction of doping concentrationinevitably increases the adversary effect of the heterointerface.

[0038] Further, the device of Japanese Laid-Open Patent Application2001-332812 suffers from the problem in that the reduction of bandgapdifference leads to decrease of reflectivity of the semiconductordistributed Bragg reflector and penetration of light into thesemiconductor distributed Bragg reflector is increased. In order tocompensate for this decrease of the reflectivity, it is necessary toincrease the number of stacks in the semiconductor distributed Braggreflector.

[0039] Conventionally, it is known in a surface-emission laser diode ofthe 0.98 μm band, and the like, to provide a hetero barrier buffer layerbetween the two layers of different Al contents and constituting thedistributed Bragg reflector in the form of a compositional graded layerhaving an Al content intermediate of the foregoing two layers forreducing the electrical resistance of the p-type semiconductordistributed Bragg reflector. Reference should be made to PhotonicsTechnology Letters Vol.2, No.4, 1990, pp.234-236 and PhotonicsTechnology Letters Vol.4, No.12, 1992, pp.1325-1327.

[0040] On the other hand, an n-type semiconductor distributed Braggreflector generally has a very low resistance as compared with a p-typesemiconductor distributed Bragg reflector, and no detailed examinationhave been made so far because it was thought that there would be littleinfluence to the device characteristic (such as device resistance of asurface-emission laser diode).

[0041] However, there occurs accumulation or depletion of carriers alsoin an n-type semiconductor distributed Bragg reflector at theheterointerface formed by two semiconductor layers of different kinds asa result of the influence of band discontinuity between two differentsemiconductor materials. Because of this, the characteristics of thedistributed Bragg reflector differ significantly from those of a bulksemiconductor. Particularly, a depletion layer characterized bydecreased carrier density forms electrostatic capacitance component, andbecome of this, restriction is imposed to electric characteristics anddevice response characteristics when a device (surface-emission laserdiode, and the like) is driven to cause pulse operation or high speedmodulation. Furthermore, because of the influence of theheterointerface, there is caused the problem in that non-linearity iscaused in the current-voltage characteristic and that thecurrent-voltage characteristic changes in response to the difference ofthe drive condition of the device.

[0042] Thus, it is necessary to conduct detailed examination regardingto the structure and electric characteristics of an n-type semiconductordistributed Bragg reflector in order to obtain a device(surface-emission laser diode, and the like) having excellentcharacteristics. Conventionally, such detailed examination was notconducted.

SUMMARY OF THE INVENTION

[0043] Accordingly, it is a general object of the present invention toprovide a novel and useful surface-emission laser diode operable in along wavelength band and an optical transmission/reception system oroptical telecommunication system that uses such a surface-emission laserdiode.

[0044] Another and more specific object of the present invention is toprovide a surface-emission laser diode or laser diode array having adistributed Bragg reflector tuned to a wavelength of 1.1 μm or longerwherein the electric resistance of the distributed Bragg reflector isminimized while maintaining high reflectance.

[0045] Another object of the present invention is to provide asurface-emission laser diode or laser diode array in which anintermediate layer is interposed between a high refractive index layerand a low refractive index layer constituting a distributed Braggreflector with a refractive index intermediate between the highrefractive index layer and the low refractive index layer, wherein thethickness of the intermediate layer is optimized for minimizing theresistance of the distributed Bragg reflector while maintaining a highreflectance.

[0046] Another object of the present invention is to provide asurface-emission laser diode or laser diode array in which anintermediate layer or heterospike buffer layer is interposed between alow refractive index layer having a wide bandgap and a high refractiveindex layer having a narrow bandgap with an intermediate bandgap,wherein the compositional profile of Al in the heterospike buffer layeris optimized so as to minimize the resistance of the distributed Braggreflector while maintaining a high reflectance.

[0047] Another object of the present invention is to provide an opticalinterconnection system or optical telecommunication system using such asurface-emission laser diode or surface-emission laser diode array.

[0048] Another object of the present invention is to provide an opticaltransmission/reception system that uses a long-wavelengthsurface-emission laser diode operable at the laser oscillationwavelength of 1.1-1.7 ,,m with low operating voltage and smalloscillation threshold current.

[0049] Another object of the present invention is to provide an opticaltransmission/reception system suitable for construction inside abuilding by using a surface-emission laser diode chip in which theoperating voltage is reduced and the threshold current for laseroscillation is reduced.

[0050] Another object of the present invention is to provide astabilized optical transmission/reception system by using along-wavelength surface-emission laser diode chip operating stably atthe wavelength of 1.1-1.7 μm for the optical source.

[0051] Another object of the present invention is to eliminate variousproblems that arise when such an optical transmission/reception systemis actually incorporated in an electronic apparatus.

[0052] Another object of the present invention is to provide a low-costand energy-saving optical transmission/reception system by using asurface-emission laser diode chip operable at low voltage with a smallthreshold current of laser oscillation.

[0053] Another object of the present invention is to facilitate theconstruction of an optical-fiber telecommunication system that uses asurface-emission laser diode operating at low voltage with lowoscillation threshold current as an optical source, by increasing thelength of the optical fiber cable extending from a module package beyonda certain length, and hence by improving the productivity of assemblingthe module package.

[0054] Another object of the present invention is to provide anoptical-fiber telecommunication system that enables optical transmissionof large capacity with low cost, by using a surface-emission laser diodehaving a reduced operational voltage and reduced oscillation threshold,as an optical source.

[0055] Another object of the present invention is to provide a reliableoptical-fiber telecommunication system by using a surface-emission laserdiode having a low operational voltage and low oscillation threshold asan optical source, such that the change of operational characteristic ofthe laser diode is suppressed and the lifetime of the laser diode isincreased.

[0056] Another object of the present invention is to provide an opticaltelecommunication system realizing excellent optical coupling between alaser diode and an optical fiber, by using a surface-emission laserdiode operable at a low operational voltage with low oscillationthreshold, for an optical source.

[0057] Another object of the present invention is to provide an opticaltelecommunication system having a simple construction characterized byreduced number of parts and is simultaneously capable of realizingexcellent optical coupling as a result of use of a surface-emissionlaser diode operable at a low operational voltage with low oscillationthreshold, for an optical source.

[0058] Another object of the present invention is to provide an opticaltelecommunication system realizing excellent optical coupling between alaser diode and an optical fiber, by using a surface-emission laserdiode operable at a low operational voltage with low oscillationthreshold, for an optical source.

[0059] Another object of the present invention is to provide an opticaltelecommunication system capable of using a laser diode without causingdamage therein, by using a surface-emission laser diode operable at alow voltage with low threshold current of laser oscillation.

[0060] According to the present invention, the surface-emission laserdiode oscillates at the wavelength band of 1.1-1.7 microns suitable foruse in an optical-fiber telecommunications in computer networks orlong-range, large-capacity telecommunication trunks. Thesurface-emission laser diode of the present invention oscillates stablyat this wavelength band with low operating voltage and low oscillationthreshold. Conventionally, such a low is surface-emission laser diodedid not exist. The laser of the present invention oscillates at theaforementioned wavelength region with low operational voltage and lowthreshold current as a result of use of an improved semiconductor Braggreflector. As a result of low power consumption, the surface-emissionlaser diode of the present invention successfully eliminates the heatingproblem. Thereby, the surface-emission laser diode of the presentinvention oscillates stably. By using such a surface-emission laserdiode, it became possible to construct a practical point-to-pointoptical transmission/reception system with low cost.

[0061] In constructing such a point-to-point opticaltransmission/reception system, the present invention avoids localizedbending of the transmission path. As a result, the opticaltransmission/reception system connects two points easily and with lowcost, without damaging the optical fiber.

[0062] Anther object of the present invention to provide a novel anduseful semiconductor Bragg reflector as well as a surface-emission laserdiode that uses such a semiconductor distributed Bragg reflector,wherein the foregoing problems are eliminated.

[0063] Another object of the present invention is to provide asemiconductor distributed Bragg reflector of low resistance and lowoptical absorption loss without sacrificing the reflectivity. Further,the object of the present invention to provide a surface-emission laserdiode, a surface-emission laser array, a surface-emission laser module,an optical interconnection system, and an optical telecommunicationsystem that uses such a semiconductor distributed Bragg reflector.

[0064] Another object of the present invention is to provide

[0065] a semiconductor distributed Bragg reflector comprising:

[0066] an alternate stacking of first and second semiconductor layershaving respective, different refractive indices; and

[0067] a plurality of intermediate layers each sandwiched between afirst semiconductor layer and a second semiconductor layer, saidintermediate layer having a refractive index intermediate between saidrefractive indices of said first and second semiconductor layers,

[0068] an intermediate layer provided in a region of said semiconductordistributed Bragg reflector having a thickness different from anintermediate layer provided in a different region of said semiconductordistributed Bragg reflector.

[0069] In a preferred embodiment of the distributed Bragg reflector, thepresent invention provides a semiconductor distributed Bragg reflectoras set forth above, wherein a difference of bandgap between said firstand second semiconductor layers is set smaller in a region of saidsemiconductor distributed Bragg reflector where said intermediate layerhas an increased thickness than in a region of said distributed Braggreflector where said intermediate layer has a reduced thickness

[0070] In a preferred embodiment of the distributed Bragg reflector, thepresent invention provides a semiconductor distributed Bragg reflectoras set forth above, wherein said intermediate layers have differentthickness and different doping concentrations within said semiconductordistributed Bragg reflector, said thickness and doping concentrationbeing changed in correspondence to electric field strength of lightwithin said semiconductor distributed Bragg reflector.

[0071] In a preferred embodiment of the semiconductor distributed Braggreflector, the present invention provides a semiconductor distributedBragg reflector as set forth above, wherein said intermediate layer hasan increased thickness and reduced impurity doping concentration in aregion of said semiconductor distributed Bragg reflector where theelectric field strength of light is large, and wherein said intermediatelayer is formed to have a reduced thickness and increased impuritydoping concentration in a region of said semiconductor distributed Braggreflector where the electric field strength of light is small.

[0072] In a preferred embodiment of the semiconductor distributed Braggreflector, the present invention provides a semiconductor distributedBragg reflector as set forth above, wherein said semiconductordistributed Bragg reflector has a design reflection wavelength of 1.1 μmor longer.

[0073] According to the present invention, it becomes possible toprovide a semiconductor distributed Bragg reflector of low resistanceand small optical absorption loss without decreasing the reflectivity ina semiconductor distributed Bragg reflector having an intermediate layerbetween two semiconductor layers of different refractive indices with arefractive index intermediate of the refractive indices of theabove-mentioned two semiconductor layers, by changing the thickness ofthe intermediate layer in a region of the semiconductor distributedBragg reflector with respect to the intermediate layers in otherregions, as explained before.

[0074] Another object of the present invention is to provide asurface-emission laser diode having a semiconductor distributed Braggreflector, said semiconductor distributed Bragg reflector comprising:

[0075] an alternate stacking of first and second semiconductor layershaving respective, different refractive indices; and

[0076] a plurality of intermediate layers each sandwiched between afirst semiconductor layer and a second semiconductor layer, saidintermediate layer having a refractive index intermediate between saidrefractive indices of said first and second semiconductor layers,

[0077] an intermediate layer provided in a region of said semiconductordistributed Bragg reflector having a thickness different from anintermediate layer provided in a different region of said semiconductordistributed Bragg reflector.

[0078] In a preferred embodiment of the surface-emission laser diode,the active layer contains a group III element of any or all of Ga and Inand a group V element of any or all of As, N and Sb.

[0079] According to the invention, it becomes possible to provide asurface-emission laser diode of reduced optical absorption loss of theoscillation light, low resistance and capable of operating at highoutput.

[0080] Another object of the present invention is to provide asurface-emission laser array comprising a surface-emission laser diode,said surface-emission laser diode having a semiconductor distributedBragg reflector comprising:

[0081] an alternate stacking of first and second semiconductor layershaving respective, different refractive indices; and

[0082] a plurality of intermediate layers each sandwiched between afirst semiconductor layer and a second semiconductor layer, saidintermediate layer having a refractive index intermediate between saidrefractive indices of said first and second semiconductor layers,

[0083] an intermediate layer provided in a region of said semiconductordistributed Bragg reflector having a thickness different from anintermediate layer provided in a different region of said semiconductordistributed Bragg reflector.

[0084] According to the present invention, it becomes possible toprovide a highly efficient surface-emission laser array of low opticalabsorption loss and low resistance and capable of operating at highoutput power.

[0085] Another object of the present invention is to provide asurface-emission laser module comprising a surface-emission laser diode,said surface-emission laser diode having a semiconductor distributedBragg reflector, said semiconductor distributed Bragg reflectorcomprising:

[0086] an alternate stacking of first and second semiconductor layershaving respective, different refractive indices; and

[0087] a plurality of intermediate layers each sandwiched between afirst semiconductor layer and a second semiconductor layer, saidintermediate layer having a refractive index intermediate between saidrefractive indices of said first and second semiconductor layers,

[0088] an intermediate layer provided in a region of said semiconductordistributed Bragg reflector having a thickness different from anintermediate layer provided in a different region of said semiconductordistributed Bragg reflector.

[0089] According to the invention, it becomes possible to provide asurface-emission laser module capable of operating at high output power,having high electric power transformation efficiency and low electricpower consumption.

[0090] Another object of the present invention is to provide an opticalinterconnection system including a surface-emission laser diode having asemiconductor distributed Bragg reflector, said semiconductordistributed Bragg reflector comprising:

[0091] an alternate stacking of first and second semiconductor layershaving respective, different refractive indices; and

[0092] a plurality of intermediate layers each sandwiched between afirst semiconductor layer and a second semiconductor layer, saidintermediate layer having a refractive index intermediate between saidrefractive indices of said first and second semiconductor layers,

[0093] an intermediate layer provided in a region of said semiconductordistributed Bragg reflector having a thickness different from anintermediate layer provided in a different region of said semiconductordistributed Bragg reflector.

[0094] According to the invention, it becomes possible to provide anoptical interconnection system of high electric power transformationefficiency and low electric power consumption.

[0095] Another object of the present invention to provide an opticaltelecommunication system having a surface-emission laser diode, saidsurface-emission laser diode having a semiconductor distributed Braggreflector, said semiconductor distributed Bragg reflector comprising:

[0096] an alternate stacking of first and second semiconductor layershaving respective, different refractive indices; and

[0097] a plurality of intermediate layers each sandwiched between afirst semiconductor layer and a second semiconductor layer, saidintermediate layer having a refractive index intermediate between saidrefractive indices of said first and second semiconductor layers,

[0098] an intermediate layer provided in a region of said semiconductordistributed Bragg reflector having a thickness different from anintermediate layer provided in a different region of said semiconductordistributed Bragg reflector.

[0099] According to the optical telecommunication system of the presentinvention, it becomes possible to provide an optical telecommunicationsystem of low electric power consumption and high electric powertransformation efficiency.

[0100] Accordingly, it is a general object of the present invention toprovide a novel and useful n-type semiconductor distributed Braggreflector as well as a surface-emission laser diode, a surface-emissionlaser array, a surface-emission laser module, an optical interconnectionsystem and an optical telecommunication system wherein the foregoingproblems are eliminated.

[0101] Another object of the present invention is to provide an n-typesemiconductor distributed Bragg reflector as well as a surface-emissionlaser diode, a surface-emission laser array, a surface-emission lasermodule, an optical interconnection system and also an opticaltelecommunication system that uses such an n-type semiconductordistributed Bragg reflector wherein electrostatic capacitance and theinfluence thereof on the current-voltage characteristic caused by thesemiconductor heterointerface is reduced.

[0102] Another object of the present invention is to provide an n-typesemiconductor distributed Bragg reflector comprising:

[0103] first and second semiconductor layers of n-type stacked with eachother, said first and second semiconductor layers having respectiverefractive indices different from each other,

[0104] wherein there is provided an intermediate layer between saidfirst and second semiconductor layers, said intermediate layer having arefractive index intermediate of the refractive indices of said firstand second semiconductor layers.

[0105] In a preferred embodiment, said intermediate layer has athickness larger than 20 [nm] in said n-type semiconductor distributedBragg reflector. In a further preferred embodiment, said intermediatelayer has a thickness equal to or larger than 30 [nm] in said n-typesemiconductor distributed Bragg reflector.

[0106] In a still further preferred embodiment, said intermediate layerhas a thickness t [nm] determined with respect to a reflectionwavelength λ [um] of said distributed Bragg reflector so as to fall inthe ranges of 20<t≦(50λ−15) [nm].

[0107] Another object of the present invention is to provide asurface-emission laser diode that uses the n-type semiconductordistributed Bragg reflector as set forth above.

[0108] Another object of the present invention is to provide asurface-emission laser diode, comprising:

[0109] an n-type semiconductor distributed Bragg reflector and a p-typesemiconductor distributed Bragg reflector disposed across an activelayer,

[0110] wherein said n-type semiconductor distributed Bragg reflector isprocessed to form a mesa.

[0111] Another object of the present invention is to provide asurface-emission laser diode comprising: an n-type semiconductordistributed Bragg reflector and a p-type semiconductor distributed Braggreflector disposed across an active layer,

[0112] said n-type semiconductor distributed Bragg reflector havingincreased resistance as compared with a region forming a cavity of saidthe surface-emission laser diode.

[0113] In a preferred embodiment, said n-type semiconductor distributedBragg reflector comprises stacking of first and second semiconductorlayers is having respective, mutually different refractive indices, saidn-type semiconductor distributed Bragg reflector further comprises anintermediate layer having a refractive index intermediate of said firstand second semiconductor layer, between said first and secondsemiconductor layers.

[0114] In a preferred embodiment, said n-type semiconductor distributedBragg reflector comprises stacking of first and second semiconductorlayers having respective refractive indices different from each other,said n-type semiconductor distributed Bragg reflector further includingan intermediate layer having a refractive index intermediate of saidrefractive indices of said first and second semiconductor layers betweensaid first and second semiconductor layers with a thick larger than 20[nm].

[0115] In a preferred embodiment, said n-type semiconductor distributedBragg reflector comprises stacking of a first and second semiconductorlayers having respective refractive indices different from each other,said n-type semiconductor distributed Bragg reflector further includingan intermediate layer having a refractive index intermediate of saidfirst and second semiconductor layers between said first and secondsemiconductor layers with a thickness of 30 [nm] or more.

[0116] In a preferred embodiment, said n-type semiconductor distributedBragg reflector comprises stacking of first and second semiconductorlayers having respective refractive indices different from each other,said n-type semiconductor distributed Bragg reflector further includingan intermediate layer having a refractive index intermediate of saidfirst and second semiconductor layers, between said first and secondsemiconductor layers with a thickness t [nm] determined with respect toa reflection wavelength λ [um] of said distributed Bragg reflector so asto fall in the ranges of 20<t≦(50λ−15) [nm].

[0117] In a preferred embodiment, said active layer is formed of a groupIII element and a group V element, said group III element of said activelayer being any or all of Ga and In, said group V element of said activelayer being any or all of As, N, Sb and P.

[0118] Another object of the present invention is to provide asurface-emission laser array comprising a surface-emission laser diodeof the type as set forth before.

[0119] Another object of the present invention is to provide asurface-emission laser module comprising a surface-emission laser diodeor a surface-emission laser array of the type as set forth before.

[0120] Another object of the present invention is to provide an opticalinterconnection system comprising a surface-emission laser diode or asurface-emission laser array or a surface-emission laser module of thetype as set forth before.

[0121] Another object of the present invention is to provide an opticaltelecommunication system comprising a surface-emission laser diode orsurface-emission laser array or a surface-emission laser module of thetype as set forth before.

[0122] According to a first aspect of the present invention it becomespossible to provide an n-type semiconductor distributed Bragg reflectorhaving reduced electrostatic capacitance at the semiconductorheterointerface with the use of the n-type semiconductor distributedBragg reflector doped to n-type and in which first and secondsemiconductor layers of different refractive indices are stacked,wherein an intermediate layer having a refractive index intermediate ofthe first and second semiconductor layers is provided between the firstand second semiconductor layers.

[0123] The influence of the heterointerface is reduced substantially bysetting the thickness of the intermediate layer to be than 20 nm, andthe current-voltage characteristic of the n-type distributed Bragg isimproved. Further, the electrostatic capacitance at the heterointerfacecan be reduced.

[0124] It should be noted that the capacitance of the heterointerface isreduced by the intermediate layer because the accumulation or depletionof the carriers at the interface is suppressed by smoothing thepotential distribution such as spike, notch, and the like, at theheterointerface. In the case that the intermediate layer is notprovided, there is caused accumulation or depletion of the carriers(electrons) due to the potential distribution such as spike, notch, andthe like, at the heterointerface, and there arises the problem ofnon-linearity in the current-voltage characteristics, such as tunnelingof the carriers through the hetero barrier, or change of thecurrent-voltage characteristics by the measurement. Furthermore, therearises the problem of formation of the electrostatic capacitance by thedepletion of the carriers. By providing the intermediate layer with thethickness of 20 nm or more to each interface of the n-type distributedBragg reflector, these problems can be improved substantially. Thus, byusing an intermediate layer thicker than 20 nm, the change of thecurrent-voltage characteristics and non-linearity of the current-voltagecharacteristics by the measurement condition are reduced substantially,and the electrostatic capacitance caused by the heterointerface isreduced also substantially. Thus, it becomes possible to obtainexcellent n-type distributed Bragg reflector with regard to electriccharacteristics.

[0125] The influence of the heterointerface is suppressed moreeffectively by setting the thickness of the intermediate layer to be 30nm or more, and the current-voltage characteristics of the n-typedistributed Bragg reflector can be improve. Further, the electrostaticcapacitance at the heterointerface can be reduced.

[0126] The reason that the capacity of the heterointerface is reduced bythe intermediate layer is because the distribution of the potential suchas spike, notch, and the like, at the heterointerface is smoothed by theintermediate layer and accumulation or depletion of the carriers at theinterface is suppressed as a result. Thicker the intermediate layer, theeffect of the intermediate layer becomes more conspicuous. By settingthe thickness of the intermediate layer to be 30 nm or more, thedifference between the CW measurement and the pulse measurement vanishesmore or less, and it becomes possible to achieve the effect mentionedbefore more effectively. Thus, the change of the current-voltagecharacteristic and the non-linearity of the current-voltagecharacteristic are suppressed more effectively. Further, theelectrostatic capacitance by the heterointerface can be reduced moreeffectively, and it becomes possible to obtain an excellent n-typedistributed Bragg reflector with regard to electrical characteristics.

[0127] The thickness t [nm] of the above-mentioned intermediate layer isdetermined in the n-type semiconductor distributed Bragg reflector withregard to the reflection wavelength λ [um] of the distributed Braggreflector so as to fall in the range of 20<t≦(50λ−15) [nm]. Because ofthis, the influence of the heterointerface is reduced while maintaininghigh reflectivity and it becomes possible to obtain an n-typedistributed Bragg reflector having excellent electric characteristics.In a distributed Bragg reflector, it is possible to obtain higherreflectivity by increasing the refractive-index difference of thesemiconductor layers constituting the distributed Bragg reflector andincreasing the steepness of the interface. Thus, there is a tendencythat the reflectivity gradually falls off with the thickness of theintermediate layer. Accordingly, excessive decrease of thickness of theintermediate layer invites sharp fall off of the reflectivity of thedistributed Bragg reflector. However, the effect of smoothing theheterointerface is enhanced with increasing thickness of theintermediate layer. Thus, it will be understood that there exists anoptimum range for the thickness of the intermediate layer in which theserequirements are satisfied simultaneously. Thus, optimum thickness rangeof the intermediate layer is chosen with respect to the reflectionwavelength λ [um] of the distributed Bragg reflector as 20<t≦(50λ−15)[nm]. With this, it becomes possible to obtain an excellent n-typedistributed Bragg reflector having excellent electrical characteristics,in which the influence of the heterointerface was reduced whilemaintaining high reflectivity.

[0128] In another aspect, there is provided a surface-emission laserdiode having the n-type semiconductor distributed Bragg reflector as setforth above. Because of this, a surface-emission laser diode havingreduced electrostatic capacitance and high speed modulation is obtained.

[0129] The surface-emission laser diode includes the n-typesemiconductor distributed Bragg reflector and the p-type semiconductordistributed Bragg reflector provided across the active layer, whereinthe n-type semiconductor distributed Bragg reflector is processed to themesa structure. Because of this, electrostatic capacitance is reducedand high speed modulation becomes possible in the surface-emission laserdiode.

[0130] It becomes possible to reduce the electrostatic capacitance andperform high speed modulation in a surface-emission laser diodecomprising an n-type semiconductor distributed Bragg reflector and ap-type semiconductor distributed Bragg reflector disposed across anactive layer, by increasing the resistance of the n-type semiconductordistributed Bragg reflector excluding the region forming a cavity of thesurface-emission laser diode.

[0131] In another aspect, there is provided a surface-emission laserdiode of the type as set forth above, wherein the n-type semiconductordistributed Bragg reflector comprises stacking of first and secondsemiconductor layers having respective, mutually different refractiveindices, wherein said n-type semiconductor distributed Bragg reflectorfurther comprises an intermediate layer having a refractive indexintermediate of said first and second semiconductor layer, between saidfirst and second semiconductor layers. As a result, the electrostaticcapacitance is reduced and a surface-emission laser diode capable ofperforming high speed modulation is provided.

[0132] In another aspect, there is provided a surface-emission laserdiode of the type as set forth above, wherein said n-type semiconductordistributed Bragg reflector comprises stacking of first and secondsemiconductor layers having respective refractive indices different fromeach other, said n-type semiconductor distributed Bragg reflectorfurther including an intermediate layer having a refractive indexintermediate of said refractive indices of said first and secondsemiconductor layers between said first and second semiconductor layerswith a thick larger than 20 [nm]. By using the construction of theforegoing embodiment, it becomes possible to reduce the area of then-type semiconductor distributed Bragg reflector contributing to theelectrostatic capacitance by confining the path of the electronscurrent, and the electrostatic capacitance of the heterointerface of then-type semiconductor distributed Bragg reflector is reduced. Thereby,high speed modulation becomes possible. Thus, as explained in relationto the effect of the invention before, the potential distribution as theheterointerface is smoothed by providing the foregoing semiconductorlayer to the heterointerface as compared with the case in which such anintermediate layer is not provided, and depletion or accumulation ofcarriers is suppressed. Thereby, the electrostatic capacitance caused bythe depletion or accumulation of the carriers can be reduceddrastically. Further, by restricting the path of the electrons in themesa region by etching and increasing of resistance of the n-typedistributed Bragg reflector as in the case of some embodiments, the areaof the distributed Bragg reflector contributing to the capacitance isreduced. Thus, by combining these features as in the case of someembodiment, the electrostatic capacitance of the device is reduced moreeffectively, and a structure very well suited for high speed modulationis obtained. The device of such an embodiment can perform high speedmodulation of 10 Gbps or more as a result of decrease of theelectrostatic capacitance of the n-type distributed Bragg reflector.

[0133] According to another aspect, there is provided a surface-emissionlaser diode of the type as set forth above, wherein said n-typesemiconductor distributed Bragg reflector comprises stacking of a firstand second semiconductor layers having respective refractive indicesdifferent from each other, said n-type semiconductor distributed Braggreflector further including an intermediate layer having a refractiveindex intermediate of said first and second semiconductor layers betweensaid first and second semiconductor layers with a thickness of 30 [nm]or more. By using the construction of the tenth embodiment, it becomespossible to reduce the area of the n-type semiconductor distributedBragg reflector contributing to the electrostatic capacitance byconfining the path of the electron current, and it becomes possible toreduce the electrostatic capacitance of the heterointerface of then-type semiconductor distributed Bragg reflector. Thereby, high speedmodulation becomes possible. Thus, as explained in relation to theeffect of the invention before, it becomes possible to smooth thepotential distribution at the heterointerface sufficiently by providingthe foregoing semiconductor layer to the heterointerface as comparedwith the case in which such an intermediate layer is not provided, anddepletion or accumulation of carriers is suppressed. Thereby, theelectrostatic capacitance caused by the depletion or accumulation of thecarriers can be reduced more drastically. Further, by restricting thepath of the electrons in the mesa region by etching and increasing ofresistance of the n-type distributed Bragg reflector as in the case ofsome embodiments, the area of the distributed Bragg reflectorcontributing to the capacitance is reduced. Thus, by combining thesefeatures as in the case of some embodiment, the electrostaticcapacitance of the device is reduced more effectively, and a structurevery well suited for high speed modulation is obtained. The device ofclaim 10 can perform high speed modulation of 10 Gbps or more as aresult of decrease of the electrostatic capacitance of the n-typedistributed Bragg reflector.

[0134] According to another aspect, there is provided a surface-emissionlaser diode of the type as set forth before, wherein said n-typesemiconductor distributed Bragg reflector comprises stacking of firstand second semiconductor layers having respective refractive indicesdifferent from each other, said n-type semiconductor distributed Braggreflector further including an intermediate layer having a refractiveindex intermediate of said first and second semiconductor layers,between said first and second semiconductor layers with a thickness t[nm] determined with respect to a reflection wavelength λ [um] of saiddistributed Bragg reflector so as to fall in the ranges of 20<t≦(50λ−15)[nm].

[0135] By using the construction of the eleventh embodiment, it becomespossible to reduce the area of the n-type semiconductor distributedBragg reflector contributing to the electrostatic capacitance byconfining the path of the electrons current, and the electrostaticcapacitance of the heterointerface of the n-type semiconductordistributed Bragg reflector is reduced. Thereby, high speed modulationbecomes possible. Thus, as explained in relation to the effect of theinvention, the potential distribution as the heterointerface is smoothedby providing the foregoing semiconductor layer to the heterointerface ascompared with the case in which such an intermediate layer is notprovided, and depletion or accumulation of carriers is suppressed.Thereby, the electrostatic capacitance caused by the depletion oraccumulation of the carriers can be reduced drastically. Further, bychoosing the thickness of the intermediate layer to the foregoing range,it becomes possible to maintain the reflectivity of the n-typedistributed Bragg reflector, and a surface-emission laser diode of lowoscillation threshold is obtained.

[0136] Further, by restricting the path of the electrons in the mesaregion by etching and increasing of resistance of the n-type distributedBragg reflector as in the case of some embodiments, the area of thedistributed Bragg reflector contributing to the capacitance is reduced.Thus, by combining these features as in the case of some embodiment, theelectrostatic capacitance of the device is reduced more effectively, anda structure very well suited for high speed modulation is obtained. Thedevice of claim 11 can perform high speed modulation of 10 Gbps or moreas a result of decrease of the electrostatic capacitance of the n-typedistributed Bragg reflector.

[0137] According to the invention, the active layer may be formed of agroup III element and a group v element, said group III element of saidactive layer being any or all of Ga and In, said group V element of saidactive layer being any or all of As, N, Sb and P in the surface-emissionlaser diode. Thus, the electrostatic capacitance is reduced and itbecomes possible to provide a surface-emission laser diode of longwavelength band and capable of performing high speed modulation.

[0138] According to another aspect of the invention, thesurface-emission laser array is formed of the surface-emission laserdiode of the type noted before. Thus, the electrostatic capacitance isreduced and it becomes possible to provide a surface-emission laserarray capable of performing high speed modulation.

[0139] According to another aspect of the invention, thesurface-emission laser module is formed of a surface-emission laserdiode of any of the type noted before or a surface-emission laser arrayof the type noted before. Thus, a surface-emission laser module capableof performing high speed optical communication and optical transmissionis provided.

[0140] According to another aspect of the invention, the opticalinterconnection system is formed of a surface-emission laser diode ofany of the types noted before or a surface-emission laser array of thetype noted before or a surface-emission laser module of the type notedbefore. Thus, an optical interconnection system capable of performinghigh speed and large capacity optical transmission is provided.

[0141] According to another aspect the invention, the opticaltelecommunication system is formed of a surface-emission laser diode ofthe types noted before or surface-emission laser array of the type notedbefore or a surface-emission laser module of the type noted before.Thus, an optical telecommunication system capable of performinghigh-speed and large capacity optical transmission is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0142]FIG. 1 is a diagram showing a cross-sectional view of along-wavelength surface-emission laser diode according to a firstembodiment of this invention;

[0143]FIG. 2 is a diagram showing an example of reflection spectrum of adistributed Bragg reflector used in the laser diode of FIG. 1;

[0144]FIG. 3 is a cross-sectional view showing the constitution of asemiconductor Bragg reflector used in the long-wavelengthsurface-emission laser diode of the first embodiment;

[0145]FIG. 4 is a diagram showing a compositional gradation of aheterospike buffer layer used in the semiconductor Bragg reflectorconstituting a part of the laser diode of the first embodiment;

[0146]FIG. 5 is a diagram showing an example of changing an Alcomposition in the heterospike buffer layer;

[0147]FIG. 6 is a diagram showing a result of evaluating adifferentiation sheet resistance of the heterospike buffer layer of FIG.4;

[0148]FIG. 7 is a diagram showing the band structure of a distributedBragg reflector near a heterojunction interface in a thermal equilibriumstate;

[0149]FIG. 8 is a diagram showing the band structure of the heterospikebuffer layer of FIG. 3 in a thermal equilibrium state;

[0150]FIG. 9 is a diagram showing an example of the band structure ofthe heterospike buffer layer used in the present invention;

[0151]FIG. 10 is a diagram showing an example of the band structure ofthe heterospike buffer layer used in the present invention;

[0152]FIG. 11 is a diagram showing an example of the band structure ofthe heterospike buffer layer used in the present invention;

[0153]FIG. 12 is a diagram showing an example of the band structure ofthe heterospike buffer layer used in the present invention;

[0154]FIG. 13 is a diagram showing the relationship between thedifferential sheet resistance of the distributed Bragg reflector and theAl compositional profile in the heterospike buffer layer;

[0155]FIG. 14 is another diagram showing the relationship between thedifferential sheet resistance of the distributed Bragg reflector and theAl compositional profile in the heterospike buffer layer;

[0156]FIG. 15 is a diagram showing another band structure of theheterospike buffer layer;

[0157]FIG. 16 is a diagram showing the relationship between thereflectance of the distributed Bragg reflector and the thickness of theheterospike buffer layer;

[0158]FIG. 17 is a diagram showing the relationship between theresistance of the distributed Bragg reflector and the thickness of theheterospike buffer layer;

[0159]FIG. 18 is another diagram showing the relationship between theresistivity of the distributed Bragg reflector and the thickness of theheterospike buffer layer;

[0160]FIG. 19 is a further diagram showing the relationship between theresistivity of the distributed Bragg reflector and the thickness of theheterospike buffer layer;

[0161]FIG. 20 is a further diagram showing another example of the bandstructure of the heterospike buffer layer;

[0162]FIG. 21 is a diagram showing the relationship between theresistivity of the distributed Bragg reflector having the heterospikebuffer layer of FIG. 20 and the thickness of the heterospike bufferlayer;

[0163]FIG. 22 is another diagram showing the relationship between theresistivity of the distributed Bragg reflector having the heterospikebuffer layer of FIG. 20 and the thickness of the heterospike bufferlayer;

[0164]FIG. 23 is a diagram showing the relationship between theresistance of various distributed Bragg reflectors and the thickness ofthe heterospike buffer layer;

[0165]FIG. 24 is another diagram showing the relationship of theresistance of various distributed Bragg reflectors and the thickness ofthe heterospike buffer layer;

[0166]FIG. 25 is a diagram showing the relationship between thereflectance of various distributed Bragg reflectors and the thickness ofthe heterospike buffer layer;

[0167]FIG. 26 is another diagram showing the relationship between thereflectance of various distributed Bragg reflectors and the thickness ofthe heterospike buffer layer;

[0168]FIG. 27 is a sectional view showing the constitution of thelong-wavelength surface-emission laser diode according to a secondembodiment of the present invention;

[0169]FIG. 28 is a diagram showing a room temperature photoluminescencespectrum of the active layer formed of a GaInNAs/GaAs double quantumwell structure;

[0170]FIG. 29 is a diagram showing a sample structure;

[0171]FIG. 30 is a diagram showing a depth profile of nitrogen andoxygen;

[0172]FIG. 31 is a diagram showing a depth profile of Al;

[0173]FIG. 32 is a diagram showing the structure obtained for a case inwhich growth interruption process is used in the form of a carrier gaspurging process;

[0174]FIG. 33 is a diagram showing a depth profile of the Al for thecase in which a growth interruption process is provided and purging isconducted by using a hydrogen gas;

[0175]FIG. 34 is a diagram showing a depth profile of nitrogen andoxygen for the case in which a growth interruption process is providedand purging is made with a hydrogen gas;

[0176]FIG. 35 is a plane view showing a wafer and a laser diode chip inwhich the long-wavelength surface-emission laser diode of this inventionis formed;

[0177]FIG. 36 is a diagram showing an example of opticaltransmission/reception system that connects an optical source and aphotodetection unit by a straight the transmission path;

[0178]FIG. 37 is a diagram showing an overall view of the above opticaltransmission/reception system;

[0179]FIG. 38 is a diagram showing an example of bending thetransmission path connecting an optical source and a photodetection unitat a right angle for avoiding an obstacle;

[0180]FIG. 39 is a diagram showing an example of the opticaltransmission/reception system of this invention that avoids an obstacleby bending the transmission path between an optical source and aphotodetection unit;

[0181]FIG. 40 is a diagram showing another example of opticaltransmission/reception system of this invention that avoids an obstaclebetween the optical source and the photodetection unit by bending thetransmission path;

[0182]FIG. 41 is a diagram showing an example of an opticaltransmission/reception system that uses a long-wavelengthsurface-emission laser diode of this invention as the optical source;

[0183]FIG. 42 is a plane view showing an example of a room in which theoptical transmission/reception system of this invention is provided;

[0184]FIG. 43 is a plane view showing an example of a room in which aconventional optical transmission/reception system is provided;

[0185]FIG. 44 is a plane view showing a further example of a room inwhich a conventional optical transmission/reception system is provided;

[0186]FIG. 45 is a schematic diagram showing an example of the opticaltransmission/reception system of this invention;

[0187]FIG. 46 is a schematic diagram showing another example of theoptical transmission/reception system of this invention;

[0188]FIG. 47 is a diagram showing an example of the electrophotographiccopying machine to which the present invention is applied;

[0189]FIG. 48 is a diagram showing an example of an electrophotographycopying machine that uses an optical transmission/reception system ofthis invention;

[0190]FIG. 49 is a diagram showing an example of an ink jet recordapparatus to which the present invention is applied;

[0191]FIG. 50 is a diagram showing an example of the ink jet recordapparatus that includes an optical transmission/reception system of thisinvention;

[0192]FIG. 51 is a diagram showing further example of the opticaltransmission/reception system of the present invention;

[0193]FIG. 52 is a diagram showing an example of optical-fibertelecommunication system to which a long-wavelength surface-emissiondiode according to an embodiment of this invention is connected;

[0194]FIG. 53 is a diagram showing a bi-directional system constructedby using the long-wavelength surface-emission laser diode of anembodiment and an optical-fiber telecommunication system connectedthereto;

[0195]FIG. 54 is a diagram showing an example of large capacityoptical-fiber telecommunication system that uses plural optical fibergroups in which the long-wavelength surface-emission laser diode of thisinvention is used;

[0196]FIG. 55 is a diagram showing a long-wavelength surface-emissionlaser diode and an optical-fiber telecommunication system connectedthereto according to an embodiment of this invention;

[0197]FIG. 56 is a diagram showing the constitution of an opticalconnection module used in an optical-fiber telecommunication systemtogether with the a wavelength surface-emission laser diode according toan embodiment of this invention;

[0198]FIG. 57 is another diagram showing the constitution of an opticalconnection module used in an optical-fiber telecommunication systemtogether with a long-wavelength surface-emission laser diode accordingto an embodiment of this invention;

[0199]FIG. 58 is a diagram showing another constitution of anoptical-fiber telecommunication system that uses a long-wavelengthsurface-emission laser diode according to an embodiment of thisinvention;

[0200]FIG. 59 is a diagram showing the constitution of anotheroptical-fiber telecommunication system that uses a long-wavelengthsurface-emission laser diode according to an embodiment of thisinvention;

[0201]FIG. 60 is a diagram explaining the constitution of an opticalconnection module used in an optical-fiber telecommunication systemtogether with a long-wavelength surface-emission laser diode accordingto an embodiment of this invention;

[0202]FIG. 61 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0203]FIG. 62 is a diagram showing length setting of optical fiber forguiding in an optical-fiber telecommunication system that uses along-wavelength surface-emission laser diode according to a firstembodiment of this invention;

[0204]FIG. 63 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode and plural optical fibers according to an embodiment of thisinvention;

[0205]FIG. 64 is a diagram showing an emission angle of an optical beamemitted from a long-wavelength surface-emission laser diode according toan embodiment of the present invention;

[0206] FIGS. 65A-65C are diagrams showing the process of fixing pluraloptical fibers with a resin according to an embodiment of thisinvention;

[0207] FIGS. 66A-66C are diagrams showing a the connect mode of along-wavelength surface-emission laser diode to plural optical fibersaccording to an embodiment of this invention;

[0208]FIG. 67 is a diagram showing possible locations of optical fibersin a closely packed state according to an embodiment of this invention;

[0209]FIG. 68 is a diagram showing an example of arranging pluraloptical fibers with a closest packed state according to an embodiment ofthis invention;

[0210]FIG. 69 is a cross-sectional view showing the constitution of anoptical fiber connected to a long-wavelength surface-emission laserdiode according to an embodiment of this invention;

[0211]FIG. 70 is a plane view showing the constitution of theoptical-fiber telecommunication system that uses a long-wavelengthsurface-emission laser diode and an optical fiber connected theretoaccording to an embodiment of this invention;

[0212]FIG. 71 is plane view showing the constitution of a bi-directionaloptical-fiber telecommunication system that uses a long-wavelengthsurface-emission laser diode and an optical fiber connected theretoaccording to an embodiment of this invention;

[0213]FIG. 72 is a plane view showing the constitution of LAN in abuilding in which the function of a long-wavelength surface-emissionlaser diode and the function of an optical fiber connected thereto areseparated;

[0214]FIGS. 73A and 73B are respectively a plan view showing along-wavelength surface-emission laser diode and an integral opticalfiber telecommunication apparatus and a cross-sectional view of anoptical waveguide according to an embodiment of this invention;

[0215]FIGS. 74A and 74B are diagrams showing the constitution of along-wavelength surface-emission laser diode chip according to anembodiment of this invention;

[0216]FIG. 75 is a diagram explaining the operation of a semiconductorlaser diode chip of FIGS. 74A and 74B;

[0217]FIGS. 76A and 76B are diagrams showing the constitution of along-wavelength surface-emission laser diode chip according to anembodiment of this invention;

[0218]FIG. 77 is a diagram explaining the operation of a laser diodechip shown in FIGS. 59A and 59B;

[0219]FIG. 78 is a diagram showing an optical transmission part of atelecommunication system in which the long-wavelength surface-emissionlaser diode according to an embodiment of this invention is used;

[0220]FIG. 79 is a diagram showing an optical transmission part of atelecommunication system in which the long-wavelength surface-emissionlaser diode of an embodiment of this invention is used;

[0221]FIG. 80 is a cross-sectional view showing an optical couplingdevice used in a long-wavelength surface-emission laser diode accordingto an embodiment of this invention;

[0222]FIG. 81 is a cross-sectional view showing the optical couplingdevice of FIG. 63;

[0223]FIG. 82 is a cross-sectional view showing an optical couplingdevice that couples with a long-wavelength surface-emission laser diodeaccording to an embodiment of this invention;

[0224]FIG. 83 is a cross-sectional view showing an optical couplingdevice that couples optically with a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0225]FIG. 84 is a cross-sectional view showing an optical fiberfixation apparatus used with a long-wavelength surface-emission laserdiode according to an embodiment of this invention;

[0226]FIG. 85 is a cross-sectional view showing the structure of anoptical coupling device;

[0227]FIG. 86 is a diagram showing a positional relationship between amonitor photodetection device and a surface-emission laser diode in anoptical-fiber telecommunication system that uses the long-wavelengthsurface-emission laser diode according to an embodiment of thisinvention;

[0228]FIG. 87 is a block diagram showing the constitution of a controlcircuit that controls output of a surface-emission laser diode accordingto an embodiment of this invention;

[0229]FIG. 88 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0230]FIG. 89 is a schematic diagram showing the constitution of anoptical-fiber telecommunication system that uses a long-wavelengthsurface-emission laser diode according to an embodiment of thisinvention;

[0231]FIG. 90 is a diagram showing a monitor photodetection device, areflection surface and a surface-emission laser diode of anoptical-fiber telecommunication system that uses the long-wavelengthsurface-emission laser diode according to an embodiment of thisinvention;

[0232]FIG. 91 is a diagram showing then constitution of an optical-fibertelecommunication system formed of a long-wavelength surface-emissionlaser diode, an optical fiber and an optical connector according to anembodiment of this invention;

[0233]FIG. 92 is a diagram showing the positional relationship betweenan optical fiber, a ferule and divided sleeves according to anembodiment of this invention;

[0234]FIG. 93 is a diagram showing the relationship between a lightemission angle and beam diameter in a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0235]FIG. 94 is a diagram showing the relationship between beamspreading and a core diameter and an optical path length of along-wavelength surface-emission laser diode according to an embodimentof this invention;

[0236]FIG. 95 is a diagram showing a calculation example of arelationship between a beam diameter and an optical path length of along-wavelength surface-emission laser diode according to an embodimentof this invention;

[0237]FIG. 96 is a diagram showing the constitution of a connection partof a laser diode and an optical fiber of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0238]FIGS. 97A and 97B are diagrams showing the constitution of aconnection part of a laser diode and an optical waveguide of anoptical-fiber telecommunication system that uses a long-wavelengthsurface-emission laser diode according to an embodiment of thisinvention;

[0239]FIG. 98 is a diagram showing the constitution of a connection partof a laser diode and an optical waveguide in an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0240]FIG. 99 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode of an embodiment of this invention in which the laser diodeand the optical fiber are coupled directly;

[0241]FIG. 100 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0242]FIG. 101 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0243]FIG. 102 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0244]FIG. 103 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0245]FIGS. 104A and 104B are diagrams showing an example of mounting along-wavelength surface-emission laser diode of an embodiment of thisinvention;

[0246]FIG. 105 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this is invention;

[0247]FIG. 106 is a block diagram showing the constitution of a controldevice used with an optical-fiber telecommunication system according toan embodiment of this invention in which a long-wavelengthsurface-emission laser diode is used;

[0248]FIG. 107 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0249]FIG. 108 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0250]FIG. 109 is a diagram showing the constitution of an optical-fibertelecommunication system that uses a long-wavelength surface-emissionlaser diode according to an embodiment of this invention;

[0251]FIGS. 110A and 110B are diagrams showing an emission angle of alaser diode according to an embodiment of this invention;

[0252] FIGS. 111A-111C are diagrams showing an optical fiber bundleaccording to an embodiment of this invention;

[0253]FIG. 112 is a diagram showing an optical fiber bundle according toan embodiment of this invention;

[0254]FIGS. 113A and 113B are diagrams showing the cross-section of anoptical fiber for multimode transmission according to an embodiment ofthis invention;

[0255]FIG. 114 is a diagram showing the cross-section of an opticalfiber for single mode transmission according to an embodiment of thisinvention;

[0256]FIG. 115 is a diagram showing the electric current-optical outputcharacteristic of a long-wavelength surface-emission laser diode of anembodiment of this invention for each temperature;

[0257]FIG. 116 is a diagram explaining the electric current control of along-wavelength surface-emission laser diode of an embodiment of thisinvention;

[0258]FIG. 117 is a diagram showing the constitution of along-wavelength surface-emission laser diode according to an embodimentof this invention that uses an electric current control;

[0259]FIG. 118 is a diagram showing the interior of an optical fibertelecommunication apparatus in which a long-wavelength surface-emissionlaser diode of an embodiment of this invention is provided;

[0260]FIG. 119 is a diagram showing the constitution of along-wavelength surface-emission laser diode module according to anembodiment of this invention;

[0261]FIG. 120 is a diagram showing the constitution of along-wavelength surface-emission laser diode according to an embodimentof this invention;

[0262]FIG. 121 is a diagram showing the constitution of along-wavelength surface-emission laser diode module according to anembodiment of this invention;

[0263]FIG. 122 is a diagram showing an example of laser chip that isused in this invention;

[0264]FIG. 123 is a diagram showing a different example of laser chipthat is used in this invention;

[0265]FIG. 124 is a diagram showing an example of system of thisinvention;

[0266]FIG. 125 is a diagram showing the constitution of such a laserchip that explains the system of this invention;

[0267]FIG. 126 is a diagram showing an example of control system of thisinvention;

[0268]FIG. 127 is a diagram showing a different example of the controlsystem of this invention;

[0269]FIG. 128 is a diagram explaining the system of this invention;

[0270]FIG. 129 is another diagram explaining the system of thisinvention;

[0271]FIG. 130 is a diagram showing the production process of a laserarray module that uses the long-wavelength surface-emission laser diodeaccording to an embodiment of this invention;

[0272]FIG. 131 is a diagram explaining the quality control process usedin the production process of FIG. 130;

[0273]FIG. 132 is a diagram showing an example of the semiconductordistributed Bragg reflector of an embodiment of the present invention;

[0274]FIG. 133 is a diagram showing an example of the linearcompositional graded layer;

[0275]FIG. 134 is a diagram showing an example of the p-typesemiconductor distributed Bragg reflector I of FIG. 1;

[0276]FIG. 135 is a diagram showing an example of the p-typesemiconductor distributed Bragg reflector II of FIG. 1;

[0277]FIG. 136 is a diagram showing the resistivity of a p-typesemiconductor distributed Bragg reflector designed for the 0.98 ,,m bandand including 4 pairs therein as a function of the thickness of thecompositional graded layer for various Al contents of the low refractiveindex layer constituting the Bragg reflector;

[0278]FIG. 137 is a diagram showing the reflectivity of a p-typesemiconductor distributed Bragg reflectors designed for the 0.98 ,,mband and including therein 5 pairs as a function of the thickness of theintermediate layer (compositional graded layer) for various Al contentsof the low refractive index layer constituting the Bragg reflector;

[0279]FIG. 138 is a diagram showing an example of the semiconductordistributed Bragg reflector of an embodiment of the present invention;

[0280]FIG. 139 is a diagram showing the construction of the p-typesemiconductor distributed Bragg reflector I of FIG. 7;

[0281]FIG. 140 is a diagram showing the construction of the p-typesemiconductor distributed Bragg reflector II of FIG. 7;

[0282]FIG. 141 is a diagram showing an example of the semiconductordistributed Bragg reflector of an embodiment of the present invention;

[0283]FIG. 142 is a diagram showing the region I of the semiconductordistributed Bragg reflector of FIG. 10;

[0284]FIG. 143 is a diagram showing the region II of the semiconductordistributed Bragg reflector of FIG. 10;

[0285]FIG. 144 is a diagram showing an example of the semiconductordistributed Bragg reflector of an embodiment of the present invention;

[0286]FIG. 145 is a diagram showing the region I of the semiconductordistributed Bragg reflector of FIG. 13;

[0287]FIG. 146 is a diagram showing the region II of the semiconductordistributed Bragg reflector of FIG. 13;

[0288]FIG. 147 is a diagram showing the reflectivity of a semiconductordistributed Bragg reflector designed for the reflection wavelength of1.3 ,,m and including 5 pairs therein as the function of the thicknessof the compositional graded layer for various Al contents of the lowrefractive index layer;

[0289]FIG. 148 is a diagram showing an example of the surface-emissionlaser diode of an embodiment of the present invention;

[0290]FIG. 149 is a diagram showing an example of the surface-emissionlaser diode produced by conducting a crystal growth process on a p-typesemiconductor substrate;

[0291]FIG. 150 is a diagram that showing an example of thesurface-emission laser array of an embodiment of the present invention;

[0292]FIG. 151 is a diagram showing an example of the surface-emissionlaser module of an embodiment of the present invention;

[0293]FIG. 152 is a diagram showing a parallel optical interconnectionsystem as an example of an embodiment of the present invention;

[0294]FIG. 153 is a diagram showing an optical LAN system as an exampleof the optical telecommunication system of an embodiment of the presentinvention;

[0295]FIG. 154 is a diagram showing an example of the n-typesemiconductor distributed Bragg reflector according to an embodiment ofthe present invention;

[0296]FIG. 155 is a diagram showing an example of the compositionalgraded layer;

[0297]FIG. 156 is a diagram showing an example of an embodiment of thepresent invention;

[0298]FIG. 157 is a diagram showing an example of an embodiment of thepresent invention;

[0299]FIG. 158 is a diagram showing an example of the n-typesemiconductor distributed Bragg reflector;

[0300]FIG. 159 is a diagram showing an example of the surface-emissionlaser diode according to an embodiment of the present invention;

[0301]FIG. 160 is a diagram showing an example of the n-typesemiconductor distributed Bragg reflector;

[0302]FIG. 161 is a diagram showing an example of realizing thesurface-emission laser diode on a p-type semiconductor substrate;

[0303]FIG. 162 is a diagram showing another example of thesurface-emission laser diode;

[0304]FIG. 163 is a diagram showing an example of the surface-emissionlaser array according to an embodiment of the present invention;

[0305]FIG. 164 is a diagram showing an example of the surface-emissionlaser diode module according to an embodiment of the present invention;

[0306]FIG. 165 is a diagram showing a parallel optical interconnectionsystem as an example of the optical interconnection system of anembodiment of the present invention;

[0307]FIG. 166 is a diagram showing an optical LAN system as an exampleof an optical telecommunication system of an embodiment of the presentinvention;

[0308]FIG. 167 is a diagram showing a current-voltage characteristic forshowing the effect of the heterointerface on the device characteristics;

[0309]FIG. 168 is a diagram showing a device including the n-typesemiconductor distributed Bragg reflector formed of AlAs/GaAs;

[0310]FIG. 169 is a diagram showing the current-voltage characteristicsof the device having a structure similar to that of FIG. 168 in which alinear composition al graded layer is provided to each AlAs/GaAsinterface of the n-type semiconductor distributed Bragg reflector;

[0311]FIG. 170 is a diagram showing the differential coefficient ofreflectivity change with regard to the thickness of the compositionalgraded layer of the distributed Bragg reflector formed of AlAs/GaAs forvarious wavelength bands;

[0312]FIG. 171 is a diagram showing an example of the n-type distributedBragg reflector of an embodiment of the present invention.

DETAILED EXPLANATION OF PREFERRED EMBODIMENTS

[0313] [First Embodiment]

[0314] First, a light-emitting device used in an optical-fibertelecommunication system of this invention will be explained withreference to FIG. 1.

[0315]FIG. 1 shows an example of a long-wavelength surface-emissionlaser diode that oscillates at the wavelength of 1.1-1.7 μm in which thetransmission loss becomes minimum.

[0316] As explained before, while there have been some suggestions aboutthe possibility of long-wavelength surface-emission laser diode thatoscillating at the wavelength of 1.1-1.7 μm, there have been noknowledge available with regard to the material and constitution for therealization such a laser diode.

[0317] This invention provides concrete constitution of such along-wavelength surface-emission laser diode that uses a material ofGaInNAs system for the active layer.

[0318] In this invention, a high refractive index layer and a lowrefractive index layer of n-type AlGaAs respectively having acomposition represented by AlxGa1-xAs (x=1.0) and AlyGal-yAs (y=0) arestacked on an n-type GaAs substrate 11 having a (100) surfaceorientation alternately and repeatedly for 35 periods with a thicknessλ/4 for each layer, wherein the λ/4 thickness is a thickness of ¼ timesthe oscillation wavelength λ of the laser diode. As a result, an n-typesemiconductor Bragg reflector (AlAs/GaAs lower semiconductor Braggreflectors) 12 is formed on the GaAs substrate 11.

[0319] Next, an n-type GaInPAs layer 13 having a composition representedas GaxIn1-xPyAs1-y (x=0.5, y=1), is provided on the Bragg reflector 12with a thickness of λ/4, In this example, this n-type GaxInl-xPyAsl-y(x=0-5, y=1) layer 13 also constitutes one of the low refractive indexlayers that forms a part of the lower part reflector 12.

[0320] Further, a lower part spacer layer 14 of undoped GaAs is formedon the GaInPAs layer 13, and an active layer 15 having a multiplequantum well structure, in which three quantum well layers 15 a eachhaving a composition represented as GaxInl-xAs are stacked on the lowerpart spacer layer 14 with a GaAs barrier layer 15 b having a thicknessof 20 nm interposed therebetween. Further, an upper part spacer layer 16of undoped GaAs is provided on the active layer 15. The active layer 15forms a resonator 15R having a thickness corresponding to one fullwavelength of oscillation wavelength of the laser diode together withthe upper and lower spacer layers 14 and 16. It should be noted that theresonator 15R constitutes the active region of the surface-emissionlaser diode.

[0321] In the constitution of FIG. 1, a p-type GaInPAs layer 17 dopedwith C (carbon) and having a composition of GaxInl-xPyAs1-y (x=0.5, y=1)is formed further on the spacer layer 16. Further, a Zn-doped GaAs layerhaving a composition of AlxGa1-xAs (x=0) and a Zn-doped AlAs layerhaving a composition represented as AlxGa1-xAs (x=1.0) are formed on thep-type GaInPAs layer 17 alternately with a thickness of ¼ times theoscillation wavelength λ in each medium, to form a periodical structure(one period).

[0322] Further, a semiconductor Bragg reflector 18 is formed thereon bystacking a C-doped, p-type AlGaAs layer having a composition representedas AlxGal-xAs (x=0.9) and a Zn-doped, p-type GaAs having a compositionrepresented as AlxGal-xAs (x=0) alternately each with a thickness of ¼times the oscillation wavelength λ of the laser diode in each medium, toform a periodic structure (25 periods). In this example, the p-typeGaInPAs layer 17 also forms one of the low refractive index layersconstituting a part of the upper part reflector.

[0323] In this embodiment, each of the upper part reflector 18 and thelower part reflector 12 has the constitution of stacking a lowrefractive index layer and a high refractive index layer alternately,wherein it should be noted that a heterospike buffer layer having acomposition represented as AlzGa1-zAs (0≦y<z<x≦1) and a refractive indexintermediate between the low refractive index layer and the highrefractive index layer is interposed in this invention between the lowrefractive index layer and the high refractive index layer as shown inFIG. 3. More precisely, in the structure of FIG. 3, the thickness of theintermediate layer, the low refractive index layer and the highrefractive index layer are set such that the change of the oscillationwavelength in the part of the intermediate layer and the high refractiveindex layer and the oscillation wavelength in the remaining part of theintermediate layer and the low refractive index respectively become λ/4.

[0324] Hereinafter, description will be made about the constitution ofthe reflector of the present invention that reflects the wavelength of1.1 ,,m or more in detail with reference to FIG. 3.

[0325]FIG. 3 shows a part of the semiconductor Bragg reflector 18. Asimilar constitution is formed also for the semiconductor Braggreflector 12. In the description below, explanation for the Braggreflector 12 will be omitted in view of the essentially sameconstitution of the reflector 18 and the reflector 12.

[0326]FIG. 2 shows an example of the reflection spectrum of thereflector in which a structural unit of AlAs/GaAs is repeated 24 times(24 pairs). In the example of FIG. 2, each of the AlAs layers is formedto have a thickness of 93.8 nm and each of the GaAs layers is formed tohave a thickness of 79.3 nm, wherein it should be noted that thesethicknesses are chosen so as to be equal to {fraction (1/4)}n wavelengthof the optical radiation that has a wavelength of 1.1 μm in the vacuumenvironment. Here, n represents the refractive index of each of the AlAslayer and the GaAs layers. By setting the thickness of the layersconstituting the distributed Bragg reflector to be equal to ¼n times ofa given wavelength λ, it is possible to tune the reflector to thiswavelength λ, and the distributed Bragg reflector thus tuned shows alarge reflectance in the wide wavelength band including the foregoingtuned wavelength λ. This wavelength λ will be referred to as designedwavelength.

[0327] It should be noted that the material of the AlGaAs system hasvarious advantages for the material of the distributed Bragg reflectorFor example, the AlGaAs material can be grown on a commonly availablelow-cost GaAs substrate with lattice matching. Further, the material hasexcellent heat radiation capability as compared with other semiconductormaterials. Furthermore, by using the material system of AlGaAs, itbecomes possible to secure a large diffractive index as compared withthe case of using other material systems. For example, a refractiveindex difference of 0.5 is realized between the end member compositionsAlAs and GaAs that constitute the AlGaAs system at the wavelength of 1.3μm. Thus, it becomes possible to achieve a large reflectance withreduced number of stacked pairs as compared with the case of using othermaterial system.

[0328]FIG. 3 shows the construction of the distributed Bragg reflectorconstituting the upper reflector 18 or lower reflector 12 of the laserdiode of FIG. 1.

[0329]FIG. 3 is referred to.

[0330] In the present embodiment, each of the upper and lower reflectors18 and 12 is formed of a distributed Bragg reflector having a reflectionwavelength of 1.1 μm or more and has a construction of stacking a lowerrefractive index layer 18 a and an upper refractive index layer 18 b,wherein it can be seen that there is provided a heterospike buffer layer18 c of AlxGa1-xAs (0≦y<z<z≦1) having a refractive index intermediatebetween the refractive index of the low refractive index layer 18 a andthe high refractive index layer 18 b.

[0331] Such a heterospike buffer layer has been studied in relation tothe laser diode of 0.85 μm band. However, it is still in the stage offeasibility study and no detailed study has been made with regard to thematerial, thickness, and like of the heterospike buffer layer. Further,there has been no proposal at all about such a heterospike buffer layerin relation to the long-wavelength surface-emission laser diode of1.1-1.7 μm band as in the case of this invention. This is because thelong-wavelength surface-emission laser diode of 1.1-1.7 μm band itselfis a new field and few researches have been made so far.

[0332] The inventor of this invention noticed the usefulness of opticaltelecommunication technology that uses a long-wavelengthsurface-emission laser diode of 1.1-1.7 μm band and devotedly conducteda study so as to realize such a laser diode.

[0333] Such a heterospike buffer layer 18 c is formed at the time offormation of the semiconductor Bragg reflector 12 or 18 by an MOCVDprocess, by controlling the source gas flow rate, such that the Alcontent in the AlGaAs film forming the heterospike buffer layer changescontinuously or stepwise. With this, the refractive index of a filmchanges also continuously or stepwise.

[0334] In more detail, the supply rate of Ga and Al is changed such thatthe value of the compositional parameter z is changed in the AlzGa1-zAs(0≦y<z<x≦1) layer from 0 to 1.0, at the time of the formation of theAlGaAs film 18 c. With this, the film composition of the heterospikebuffer layer changes gradually from GaAs to AlGaAs to AlAs. Such achange of the supply rate is caused by controlling the gas flow rate atthe time of the formation of film 12 c as noted before. A similar effectis obtained when the ratio of Al and Ga is changed stepwise orcontinuously.

[0335] The reason that such a heterospike buffer layer is provided is toeliminate the problem of increased resistance, which appears in asemiconductor Bragg reflector, particularly in a p-type semiconductorBragg reflector such as the Bragg reflector 18. Such increase ofelectric resistance is caused as a result of hetero barrier formed atthe hetero interface where two different semiconductor layers of thesemiconductor Bragg reflector make a contact. By changing the Alcomposition gradually at the hetero interface from the low refractiveindex layer to the high refractive index layer as in the case of thisinvention, there is also caused a corresponding gradual change ofrefractive index at the hetero interface, in this way, the occurrence ofthe hetero barrier at the hetero interface is successfully suppressed.

[0336] Hereinafter, more detailed explanation will be made about such aheterospike buffer layer with reference to FIG. 4.

[0337]FIG. 4 shows an example of the semiconductor Bragg reflector 18provided with the heterospike buffer layer 18 c between twosemiconductor layers 18 a and 18 b constituting the semiconductor Braggreflector. It should be noted that FIG. 4 shows the case of using thesemiconductor material AlzGal-zAs (0≦y<z<x≦1) of the AlGaAs system forthe material of the semiconductor Bragg reflector.

[0338] The two semiconductor layers 18 a and 18 b constituting thesemiconductor Bragg reflector 18 of FIG. 4 are AlAs and GaAs, and acompositional gradation layer changing the Al content gradually thereinis provided between the layers 18 a and 18 b as the heterospike bufferlayer. The heterospike buffer layer thus formed has valence band energyintermediate between the valence band energy of AlAs and the valenceband energy of GaAs. Thus, the ratio of Al to Ga is changed from GaAs toAlGaAs to AlAs in the heterospike buffer layer, and the value of z ofthe AlzGa1-zAs (0≦y<z<x≦1) heterospike buffer layer is changed graduallyfrom 0 to 1.0.

[0339] In the semiconductor material of the AlGaAs system, the bandgapenergy increases with the Al content and the refractive index falls offwith the Al content. In the conduction band, there occurs an increase ofband energy until the Al content x reaches 0.43, and the band energystarts to decrease thereafter. In the valence band, on the other hand,the valence band energy falls off monotonously with the increment of theAl content x, In total, the bandgap energy increases with the Al contentx.

[0340] In the case of the quaternary system of

[0341] AlGaInP, a similar trend as in the case of increasing the Alcontent in the AlGaAs system appears with increase of the AlInPcomposition, and the conduction band energy increases up to the point inwhich the AlInP composition has reached 0.7. Thereafter, the conductionband energy starts to decrease. On the other hand, the valence bandenergy decreases monotonously with the increase of AlInP composition.

[0342] In the example of FIG. 4, it should be noted that the rate of thecompositional gradation (rate of increase of bandgap energy) is setlarger in the region near the GaAs layer (region I in FIG. 4) ascompared with the region near the AlAs layer (region II in FIG. 4). Forthe purpose of comparison, a structure having a linear compositionalgradation layer, in which the Al content therein is changed onlylinearly, for the heterospike buffer layer 18 c is shown in FIG. 5.

[0343]FIG. 6 shows the result of evaluation of the electric resistanceof the p-type distributed Bragg reflector 18 tuned to the reflectionwavelength of 1.3 μm, wherein the Bragg reflector 18 is formed byrepeating the AlAs/GaAs unit structure, including therein theheterospike buffer layer of 20 nm thickness between the AlAs layer 18 aand the GaAs layer 18 b, four times (four pair stacking).

[0344] In FIG. 6, each of the layers 18 a-18 c of the distributed Braggreflector 18 including the heterospike buffer layer 18 c is formed of ap-type layer having a career density of 1×10¹⁸ cm⁻³, and the verticalaxis of FIG. 6 represents the value of differential sheet resistance atnear zero bias state. On the other hand, the horizontal axis of FIG. 6represents the Al compositional gradation in the region I. It should benoted that the “Al compositional gradation” is defined as the change ofthe Al content in the region I divided by the thickness of the region I.Thus, FIG. 6 represents the case in which the thickness of the region Iis changed variously, while it should be noted that the total thicknessof the region I and the region II is maintained constant at 20 nm. Thus,the thickness and the compositional gradation of the region II aredetermined by the thickness and the compositional gradation of theregion I. The Al compositional gradation for the case a simple linearcompositional gradation layer is provided between the GaAs layer and theAlAs layer becomes 0.05 nm⁻. This hits the A point of the drawing.

[0345] From FIG. 6, it can be seen that the resistance is reducedfurther by increasing the Al compositional gradient in the heterospikebuffer layer 18 c from the region II to the region I as compared withthe case of FIG. 5 in which the compositional gradation is linearthroughout the heterospike buffer layer 18 c. Also, it can be seen thatthere exists an optimum Al compositional gradation in which theresistance becomes minimum. For example, in the case the thickness ofthe region I is 10 nm (the same thickness as the region II), it can beseen that the resistance is reduced to about 80% of the conventionalresistance value obtained for the case in which the Al compositionalgradation is set to 0.09 nm⁻¹. This trend does not change depending onthe applied voltage.

[0346] Next, the reason of this will be explained.

[0347]FIG. 7 shows the valence band structure of the distributed Braggreflector having the AlAs/GaAs structure for the part near the heterosurface in thermal equilibrium state.

[0348]FIG. 7 is referred to.

[0349] It can be seen that the heterospike originating from the banddiscontinuity appear predominantly at the side of the widegap AlAslayer. In the side of the GaAs layer, the occurrence of the notch istrifling. Thus, the notch at the side of the GaAs layer does not becomethe cause of increase of resistance. Therefore it is concluded that, forthe reduction of resistance of a distributed Bragg reflector, it isimportant to eliminate or reduce the heterospike appearing at the sideof the AlAs layer within a limited thickness of the heterospike bufferlayer.

[0350] In the structure of FIG. 7, it can be seen that the Al content ofthe heterospike buffer layer 18 c is increased sharply at the side ofthe GaAs layer 18 b in which there occurs the notch formation, whilethis corresponds to a gentle compositional gradient at the side of theAlAs layer, in side which the remarkable heterospike formation takesplace. By doing so, it becomes possible to reduce the spike formation ascompared with the case of changing the composition of the heterospikebuffer layer linearly as in the case of FIG. 5. When the Alcompositional gradation is set smaller in the region I with regard tothe region II, on the other hand, there occurs an unwanted increase ofresistance.

[0351]FIG. 8 shows the valence band diagram of the structures of FIGS. 3and 4 in a thermal equilibrium state, wherein the continuous linerepresents the band structure of FIG. 3 while the dotted line representsthe band structure of FIG. 4.

[0352]FIG. 8 is referred to. By using the compositional gradationprofile of FIG. 4, it becomes possible to reduce the compositionalgradation of the heterospike buffer layer 18 c in the region II at theside of the AlAs layer 18 a with regard to the region I at the side ofthe GaAs layer 18 b as compared with case of using the simplecompositional gradation of FIG. 5 with the same thickness. Thus, it isconcluded that the resistance of the semiconductor distributed Braggreflector is reduced by increasing the compositional gradation in theregion I of the heterospike buffer layer I.

[0353] In the example of FIG. 4, it can be seen that the heterospikebuffer layer 18 c is formed of two regions I and II each having a linearAl compositional profile. On the other hand, it is also possible tochange the Al content non-linearly as represented in FIG. 9. Even insuch a case, it is possible to define the boundary between the regions Iand II as an intersection of the intercept drawn to the valence band atthe boundary between the GaAs layer 18 b and the heterospike bufferlayer 18 c and the intercept drawn also to the valence band at theboundary between the AlAs layer 18 a and the heterospike buffer layer 18c, as represented in FIG. 9.

[0354] In the heterospike buffer layer 18 c, it is not necessary thatthe Al composition changes continuously but another layer may existbetween the regions I and II as represented in FIG. 10.

[0355] According to FIG. 6, the differential sheet resistance of theBragg reflector is reduced with decreasing thickness of the region I.Further, it can be seen that the minimum of the differential sheetresistance is achieved when there is no region I provided as representedin FIG. 11 or when the thickness of the region I is small enough that itcan be regarded that there is a discontinuity in the conduction bandbetween the GaAs layer 18 b (narrow gap layer) and the heterospikebuffer layer 18 c (widegap layer) as represented in FIG. 12.

[0356]FIG. 13 shows the differential sheet resistance of the distributedBragg reflector similar to the one shown in FIG. 6 as a function of theAl compositional gradation in the region II. It should be noted that theAl compositional gradation in the region II is determined uniquely fromthe Al compositional gradation in the region I and the thickness of theregion I.

[0357] From FIG. 13, it can be seen that there appears a minimum of thedifferential sheet resistance at a particular value of the compositionalgradation in the layer 11, wherein this particular value of thecompositional gradation is irrelevant to the thicknesses of the regionsI and II.

[0358]FIG. 14, on the other hand, shows a similar relationship as thecase of FIG. 13 for the case the total thickens of the regions I and 11is 40 nm.

[0359] Referring to FIG. 14, it can be seen that a similar minimum ofresistance appears at a particular value of the Al compositionalgradation.

[0360] From FIGS. 13 and 14, it is possible to evaluate the optimumdiscontinuity of valence band energy between the narrow gap layer (GaAslayer 18 b of FIG. 3) and the heterospike buffer layer 18 c forminimizing the resistance of the distributed Brag reflector having aheterospike buffer layer.

[0361] As noted above, in the semiconductor distributed Bragg reflectorhaving a heterospike buffer layer, it is possible to minimize theresistivity of the reflector by changing the Al composition of theheterospike buffer layer sharply in the region I and optimizing the Alcompositional gradation in the region II correspondingly, such thatthere appears a discontinuity or substantial discontinuity at theboundary between the narrow gap layer 18 b and the heterospike bufferlayer 18 c.

[0362] For example, it is possible to reduce the resistance of thedistributed Bragg reflector by about 75% by reducing the thickness ofthe region I to 1 nm in the heterospike buffer layer 18 c having athickness of 20 nm, as compared with the distributed Bragg reflectorhaving a heterospike buffer layer of the same thickness except that theAl composition is changed linearly throughout the heterospike bufferlayer.

[0363] In this case, too, it is not necessary that the Al contentchanges linearly in the regions I and II of the heterospike buffer layer18 c but the Al content may be changed non-linearly as represented inFIG. 15.

[0364] Further, it should be noted that the foregoing consideration isnot limited to the distributed Bragg reflector of the AlGaAs system butis applicable also to the distributed Bragg reflector of other materialsystems such as AlGaInP system. In the case of the distributed Braggreflector of the AlGaInP system, it is possible to achieve a similareffect by changing the AlInP composition in the heterospike bufferlayer.

[0365] The present invention has a feature in that the main layer 18 aor 18 b and the heterospike relaxation layer 18 c constituting thesemiconductor distributed Bragg reflector of FIG. 3, have a carrierdensity in the range from 5×10¹⁷ cm⁻³ to 2×10¹⁸ cm⁻³, the heterospikerelaxation layer 18 c has a thickness in the range from 5 nm to 40 nm,and the average change rate of Al composition in the region II (see FIG.4) is in the range from 0.02 nm⁻¹ to 0.15 nm⁻¹. Here, the Alcompositional gradient in the region I is defined as “Al compositionalgradient={variation (0-1) of Al content in the region I}/dI”. Bychoosing each parameter of the heterospike buffer layer 18 c thatincludes the distributed Bragg reflector to the foregoing range,reduction of resistance is achieved easily and effectively.

[0366] Table 12 below shows the Al composition gradient that providesminimum of the electric resistance and the corresponding sheetdifferential resistance for the case of changing the carrier density ofthe distributed Bragg reflector and the heterospike buffer layer 18 c(5×10¹⁷ cm⁻³, 2×10¹⁸ cm⁻³) and the thickness of the heterospike bufferlayer 12 in the structure of FIG. 9, together with the percentage of theelectric resistance decrease in comparison with the case in which asimple linear compositional gradient is used for the heterospike bufferlayer (the structure of FIG. 5). TABLE 12 Heterospike buffer layer 5 ×10¹⁷[cm⁻³] 2 × 10¹⁸[cm⁻³] thickness carrier density carrier density  5[nm] 0.16[nm⁻¹]/8.4 × 10⁻⁶[Ωcm²]/ 0.16/4.5 × 10⁻⁸[Ω cm²]/ 83% 90% 40[nm] 0.02[nm⁻¹]/2.1 × 10⁻⁹[Ωcm²]/ — 91%

[0367] In view of increase of electrical resistance with decrease of thecarrier density, the value of 5×10¹⁷ cm⁻³ is chosen as the actuallyallowable lower limit. Further, a value of 2×10¹⁸ cm⁻³ is chosen as theallowable upper limit in view of conspicuous optical absorptionparticularly in the case of a p-type semiconductor.

[0368] In the case the thickness of the heterospike buffer layer 12 isincreased, a remarkable decrease of resistance is achieved. On the otherhand, such a decrease of the thickness of the heterospike buffer layer12 is not preferable in view of decrease of reflectance of thedistributed Bragg reflector. From the viewpoint of reflectance, it isbelieved that the value of 40 nm or less is important for the practicalthickness of the heterospike buffer layer.

[0369] In the case the thickness is too small, the desired resistancedecrease is not attained. Thus, it is believed that the value of 5 nm ormore is important for the thickness of the heterospike buffer layer 12for realizing sufficient resistance reduction effect.

[0370] As compared with the case of the simple compositional gradationlayer in which the Al content is changed linearly from the small-bandgaplayer to the large-bandgap layer constituting the main layers of thedistributed Bragg reflector, the foregoing construction can achievefurther reduction of the resistance. Within the foregoing range, thedifferential sheet resistance is decreased to about 75% (1.2×10⁻⁹ Ωcm²in terms of differential sheet resistance) in the embodiment of claim 3,and thus a significant effect is achieved.

[0371] Thus, the present embodiment can reduce the resistance further ascompared with the case of using the linear compositional gradient in theheterospike buffer layer 18 c of the same thickness. In the case ofachieving the same resistance value, on the other hand, the presentembodiment allows the use of reduced thickness for the necessaryheterospike buffer layer 18 c. Thus, adversary effect on the opticalproperties such as reflectance is minimized.

[0372] Thus, it becomes possible to obtain a distributed Bragg reflectorexcellent in terms of electric properties and optical properties, bychoosing the structure of the distributed Bragg reflector andheterospike buffer layer as set forth in the claims.

[0373] Next, the optimum thickness of such a heterospike buffer layerAlzGa1-zAs (0≦y<z<x≦1) will be described.

[0374]FIG. 16 shows the relationship between the reflectance andthickness of the heterospike buffer layer for a distributed Braggreflector tuned to the 0.88 μm band and 1.3 μm band for the case theheterospike buffer layer 18 c has a linear compositional profile of Alas represented in FIG. 5. The reason that the reflector tuned to theconventional wavelength of 0.88 μm was prepared in this experiment is tomake a comparison with a conventional reflector tuned to thiswavelength. It should be noted that GaAs absorbs optical radiationshorter than 0.87 μm. In FIG. 16, GaAs is used for the high refractiveindex layer and AlAs is used for the low refractive index layer.

[0375] The distributed Bragg reflector thus produced needed 18 pairs ormore of the low refractive index layer and the high refractive indexlayer in order to achieve the reflectance exceeding 99.9% at thewavelength band of 0.88 μm, while 23 pairs or more were needed forachieving the same reflectance at the wavelength band of 1.3 μm. Itshould be noted that FIG. 16 represents the relationship between thereflectance and thickness of the heterospike buffer layer of thedistributed Bragg reflector tuned to the respective wavelengths.

[0376] Table 1 summarizes the reflectance of FIG. 16. TABLE 1 0 nm 5 nm10 nm 0.88 μm band 99.914 99.912 99.905  1.3 μm band 99.923 99.92399.920

[0377] Referring to Table 1, it can be seen that there is no decrease ofreflectance in the 1.3 μm band until the thickness of the heterospikebuffer layer 18 c reaches 5 nm. In the 0.88 μm band, on the other hand,there starts a decrease of reflectance at the thickness of 5 nm of theheterospike buffer layer 18 c. In the case of a surface emission laserdiode, the cavity length is characteristically small and the effect ofreflection loss by the mirror appears conspicuously. Thus, even in thecase the decrease of the reflectance is very small, a large effect iscaused in the threshold current of the laser diode.

[0378]FIGS. 17 and 18 show the resistivity of the AlAs/GaAs distributedBragg reflector, having the heterospike buffer layer 18 c similar toFIG. 5 and tuned to the reflection wavelength of 1.3 μm, near the zerobias point. In the drawings, the resistivity is represented in terms ofΩcm² and is defined as dV/dJ (V: voltage represented in terms of volts;J: current density represented in terms of A/cm²). In the illustratedexample, the number of pairs is four. It should be noted that FIG. 17and FIG. 18 shows essentially the same diagram except that FIG. 17 showsthe diagram in the logarithmic representation while FIG. 18 shows thediagram in the linear scale representation. In FIGS. 17 and 18, thebroken line represents the resistivity evaluated from the bulk crystalresistivity, neglecting the effect of the band discontinuity. Further,each layer of the distributed Bragg reflector is doped to the p-typewith the carrier density of 1×10¹⁸ cm⁻³.

[0379] Referring to FIG. 17, it can be sent hat the resistivity of thedistributed Bragg reflector decreases with the thickness of theheterospike buffer layer 18 c. In the case of the reflector tuned to thewavelength of 1.3 μm, the proportion of the compositional gradationlayer 18 c with regard to the rest of the layers 18 a and 18 b isrelatively small. Thus, it becomes possible to reduce the resistivity bytwo orders by providing the heterospike buffer layer 18 c with thethickness of 5 nm, without causing influence to the reflectance. Longerthe reflection wavelength, the heterospike buffer layer 18 c can beformed with increased thickness. When the thickness is smaller than 5nm, on the other hand, the desired reduction of the resistance is notobtained as can be seen from FIG. 17.

[0380] As represented in FIG. 17, the distributed Bragg reflector showsa very high resistivity value of 1 Ωcm² when the heterospike bufferlayer 18 c is omitted. Thus, in the matter of practical problems, it isextremely difficult to drive a laser diode having a distributed Braggreflector in which 20 or more stacks are provided. In order to drivesuch a laser diode, a very high voltage would be needed. Thus, it isdifficult to apply such a distributed Bragg reflector to a currentdriven device such as a surface-emission laser diode.

[0381] In the case of providing the heterospike buffer layer 18 c withthe thickness of 5 nm as noted above, the resistance of the reflector isreduced by two orders as compared with the case not providing such aheterospike buffer layer. As a result, supply of the drive current tothe laser diode is facilitated and laser oscillation becomes possible.Further, the necessary drive voltage is reduced, and the problems suchas failure or malfunctioning of the laser diode or reliability areresolved. As represented in Table 1, there is no decrease ofreflectance. Thus, the laser oscillation can be achieved at a lowthreshold current density.

[0382] Thus, the foregoing thickness of 5 nm is thought as being thelower limit of the heterospike buffer layer that enables decrease ofresistance without causing problems to the reflection performance atlong wavelengths. Therefore, the heterospike buffer layer 18 c should beformed with a thickness of 5 nm or more.

[0383] By increasing the thickness of the heterospike buffer layer 18 c,the resistivity falls of sharply, and with this, the drive voltage andthe heat generation of the device are reduced. Associated with this, theoperational temperature range of the laser diode is extended and thelaser output is increased.

[0384] In the case of allowing 99.8% reflectance for the distributedBragg reflector, the maximum thickness of the heterospike buffer layerthat can be provided to the distributed Bragg reflector tuned to the0.88 μm wavelength is limited to 20 nm or less. In the case of thedistributed Bragg reflector tuned to the 1.3 μm, it is possible toprovide the heterospike is buffer layer 18 c up to the thickness of 50nm.

[0385] As represented in FIG. 6, the resistance of the distributed Braggreflector decreases with the thickness of the heterospike buffer layer18 c up to the thickness of 50 nm for the heterospike buffer layer 18 c,and a resistivity of 1.05 times the bulk resistivity is achieved at thethickness of 50 nm for the layer 18 c. When the thickness of theheterospike buffer layer 18 c is increased further, on the other had,there appears a saturation in the decrease of the resistivity.

[0386] It should be noted that the reflectance of the distributed Braggreflector starts to decrease sharply with increase of thickness of theheterospike buffer layer 18 c, and the reflectance is reduced to thelevel of 99.8% or les when the thickness of the layer 18 c has exceeded50 nm. Thus, it is necessary to limit the thickness of the heterospikebuffer layer 18 c to be 50 nm or less in order to satisfy therequirement of high reflectance and low resistivity simultaneously,

[0387]FIG. 19 shows the reduction of the reflectance in detail, whereinFIG. 19 shows the change rate of the reflectance R (|dR/dt|) with regardto the thickness t of the heterospike buffer layer 18 c.

[0388] Referring to FIG. 19, it can be seen that there occurs a sharprise of the reflectance when the thickness of the heterospike bufferlayer has exceeded 50 nm. With this, there occurs an increase of thethreshold current of laser oscillation.

[0389] Thus, in the distributed Bragg reflector tuned to the wavelengthof 1.3 μm and having a heterospike buffer layer having a thickness of 5nm or more but 50 nm or less, it is possible to reduce the resistancecaused by the heterointerface while maintaining a high reflectance. Byusing such a distributed Bragg reflector in a surface-emission laserdiode, it is possible to achieve a laser oscillation under a practicaldrive condition.

[0390] In the case of a surface-emission laser diode, it is necessary todesign the reflector such that the distributed Bragg reflector 18 at theexit side has a relatively smaller reflectance for facilitating opticaloutput in order to increase the output power of the laser diode.Further, in order to obtain a stable laser oscillation up to the highoutput region (high level injection region), it is necessary to set theoutput saturation point as high as possible by suppressing the deviceheat generation. The distributed Bragg reflector having a relativelythick (50 nm) heterospike buffer layer 18 c of the present inventionsatisfies these conditions and is suited for use in a high-power laserdiode.

[0391] Thus, according to the long-wavelength surface-emission laserdiode of the present invention, it is possible to optimize thereflectance and electric properties of the distributed Bragg reflectorused therein by optimizing the thickness of the heterospike buffer layer18 c within the range of 5-50 nm.

[0392] In the example of FIG. 3, the low refractive index layer 18 a isformed of an AlAs layer and the high-refractive index layer 18 b isformed of a GaAs layer. On the other hand, it is also possible to usetwo AlGaAs layers for the low-refractive index layer 18 a and the highrefractive index layer 18 b, by changing the Al content in the AlGaAslayers. In this case, it is preferable to increase the difference of theAl content between the low-refractive index layer 18 a and the highrefractive index layer 18 b for decreasing the number of stacks in thereflector, as the reflectance of the reflector is increased withincreasing degree of difference of the refractive index between the lowrefractive index layer and the high refractive index layer. Thus, itwill be noted that the construction of FIG. 3 provides the maximumrefractive index difference between the low refractive index layer andthe high refractive index layer by using GaAs and AlAs.

[0393] As noted already, in the heteroepitaxial structure in which thereis a large difference of Al content between the low refractive indexlayer and the high refractive index layer, the band discontinuity at thevalence band is also increased and there arises a problem of increaseddevice resistance in trade off to the increase of the reflectance. Thus,in such a case, it is necessary to provide the heterospike buffer layerof sufficiently large thickness between the high refractive index layer18 b and the low refractive index layer 18 a for reducing theresistivity of the reflector. However, use of such a heterospike bufferlayer has been difficult in the conventional distributed Bragg reflectortuned to the wavelength band of 0.85 μm. In the present invention, onthe contrary, it is possible to achieve a high reflectance andsimultaneously a low resistance while using the material system ofGaAs/AlAs for the distributed Bragg reflector.

[0394] In the distributed Bragg reflector of FIG. 3, it is possible toform the heterospike buffer layer 18 c in the range of 20-50 nm in thecase the reflector is tuned to the reflection wavelength of 101 μm ormore.

[0395] Referring to FIG. 17 again in more detail, it can be seen thatthe resistivity of the distributed Bragg reflector decreases sharplywith increase of the thickness of the heterospike buffer layer 18 c andthen approaches gradually to the bulk resistivity. In the example ofFIG. 17, the threshold thickness of the heterospike buffer layer 18 cabove which the saturation of resistivity takes place is about 20 nm. Atthis thickness of 20 nm, it can be seen that the resistivity is reducedto about twice the bulk resistivity. Thus, by using the thickness of 20nm or more but 50 nm or less, it is possible to obtain a distributedBragg reflector having a resistivity comparable to the bulk resistivity.

[0396]FIG. 20 shows a band structure for another distributed Braggreflector.

[0397] In the example of FIG. 20, GaAs is used for the high-refractiveindex layer 18 b in the structure of FIG. 3 while the low refractiveindex layer 18 a formed of Al_(0.8)Ga_(0.2)As. Further, the example ofFIG. 20 uses a compositional gradation layer having a thickness of 30 nmfor the heterospike buffer layer 18 c. In the example of FIG. 20, thecompositional gradation layer is formed such that the valence bandenergy changes parabolic with the thickness. In such a paraboliccompositional gradation layer, the valence band energy has a downwardlyconvex shape.

[0398] In the construction of FIG. 20, it should be noted that thedesign wavelength (λ) of the distributed Bragg reflector is 1.5 μm, andthe thickness of the Al0.8Ga0.2As layer 18 a and the thickness of theGaAs layer 18 b are respectively chosen to 110.8 nm and 125.5 nm so asto be equal to λ/4n, wherein n is a refractive index in the respectivelayers.

[0399] In the distributed Bragg reflector of FIG. 20, each of thesemiconductor layers is formed to have a thickness subtracted with theoptical thickness of the parabolic compositional gradation layer 18 cfrom the above noted thickness. In the case an Al_(0.6)Ga_(0.4)As layeris used for the low-refractive index layer 18 a as in the case of theembodiment to be described later, the λ/4n thickness of theAl_(0.6)Ga_(0.4)As layer for the distributed Bragg reflector tuned tothe 1.5 μm wavelength becomes 121.7 nm.

[0400] In the present embodiment, each of the Al0.8Ga0.2As layer 18 a,the GaAs layer 18 b and the parabolic compositional gradation layer 18 cis doped to the p-type such that each of the foregoing layers have auniform carrier density of 5×10¹⁷ cm⁻³.

[0401] Thus, in the present embodiment, the resistivity of thedistributed Bragg reflector is reduced to the value generally equal tothe bulk resistivity in spite of the fact that the distributed Braggreflector is doped to a relatively small doping density of 5×10¹⁷ cm⁻³due to the use of the relatively thick compositional gradation layer 18c (50 nm). The use of such a low doping density is also advantageous inview of reduced optical absorption caused by the transition between thevalence bands. It should be noted that, in such a distributed Braggreflector tuned to the longer wavelength of 1.5 μm, it is easy toprovide a thick parabolic compositional gradation layer 18 c whilemaintaining high reflectance.

[0402] While a compositional gradation layer having a parabolicgradation is used for the heterospike buffer layer 18 c in theembodiment of FIG. 20, the heterospike buffer layer 18 c may be formedof another layer. Further, the distributed Bragg reflector may be tunedto the wavelength other than 1.5 μm. Similarly, the Al composition ofthe low-refractive index layer may be different from the one explainedabove. Further, the doping density may be changed in each of the layersin the distributed Bragg reflector.

[0403] As noted above, the resistance of the distributed Bragg reflectortuned to the long wavelength band of 1.1 μm or more is reduced sharplywhen the thickness of the heterospike buffer layer 18 c is increased asis represented in FIG. 17, until the thickness of the heterospike bufferlayer 18 c exceeds a predetermined thickness. This predeterminedthickness of the heterospike buffer layer 18 c within which the sharpchange of the resistivity takes place is related to the doping densityused in the distributed Bragg reflector.

[0404]FIG. 21 shows the case the doping density of 7×1017cm−3 is used inthe AlAs/GaAs distributed Bragg reflector of FIG. 20 for each of thelayers therein.

[0405] Referring to FIG. 21 in which the doping density is reduced, itcan be seen that the thickness range of the heterospike buffer layer 18c in which there occurs a sharp drop of the resistivity in thedistributed Bragg reflector has been increased to 30 nm. When theforegoing limit is exceeded, it can be seen that the resistivity ischanged linearly to the value of the bulk resistivity. Particularly, inthe case of a p-type distributed Bragg reflector, there occurs anincrease of optical absorption when the hole density (doping density) isincreased as a result of optical absorption between the valence bands inaddition to the free carrier absorption. Thus, when such a distributedBragg reflector is applied to a laser diode, there is caused an increaseof the laser threshold current.

[0406] Thus, from the viewpoint of optical absorption, it is preferableto use a low carrier density. As the effect of the transition betweenthe valence bands appears conspicuous for optical radiation of longwavelengths, it is important to suppress the absorption loss in the longwavelength laser diode operating at the wavelength of 1.1 μm or more. Inthe case of a conventional distributed Bragg reflector having a dopingdensity exceeding 1×10¹⁸ cm⁻³, it is difficult to reduce the absorptionloss.

[0407] Because of this reason, there are cases in which the dopingdensity is reduced to 1×10¹⁸ cm⁻³ or less in one of the low refractiveindex layer 18 a, the high refractive index layer 18 b, the heterospikebuffer layer 18 c, or all of the layers 18 a-18 c. In such a distributedBragg reflector in which the doping density is reduced, however, thereoccurs an increase of extension of the depletion layer, and the problemof increase of resistance is inevitable. When the doping density is thusreduced, therefore, it is necessary to increase the thickness of theheterospike buffer layer, while it is noted that such a compensatingeffect of the heterospike buffer layer appears when the thicknessthereof has reached 30 nm or more. In the case the doping density isreduced further, a more thick heterospike buffer layer 18 c is needed.In such a case, the effect of the hetero interface is compensated for byusing a larger thickness for the heteroepitaxial buffer layer within theforegoing range, such as 40 nm or 50 nm. A similar argument applies alsoto the case when AlGaAs is used in place of AlAs in the distributedBragg reflector.

[0408]FIG. 22 shows the result of calculation of resistivity of thedistributed Bragg reflector including 4 pairs of theAl_(0.8)Ga_(0.2)As/GaAs structure. In the example of FIG. 22, the dopingdensity of each layer is reduced further to the level of 5×10¹⁷ cm⁻³.

[0409] In this case, too, the heterospike buffer layer 18 c has athickness of 30 nm or more and it can be seen that the resistivity ofthe distributed Bragg reflector is comparable to the resistivity of thebulk resistivity. In the case of a distributed Bragg reflector in whichany or all of the high refractive index layer 18 a, low refractive indexlayer 18 b and the heterospike buffer layer 18 c are set smaller thanthe conventional doping density of 1×10¹⁸ cm⁻³, for example, thereappears a saturation of resistivity when the thickness of theheterospike buffer layer 18 c has reached 30 nm. Thus, in the case atleast one of the layers constituting the distributed Bragg reflector hasthe doping density of 1×10¹⁸ cm⁻³ or less, it is possible to decreasethe resistance effectively by using the heterospike buffer layer havinga thickness in the range of 30-50 nm.

[0410] Of course, the foregoing thickness of the heterospike bufferlayer 18 c is effective also in the distributed Bragg reflector having alarger doping density and may be used in the case all of the layersconstituting the distributed Bragg reflector has a doping density of1×10¹⁸ cm⁻³ or more. On the other hand, it is especially advantageous touse the doping density of 1×10 ¹⁸ cm⁻³ or less in combination with theheterospike buffer layer having a thickness appropriately chosen fromthe range of 30-50 nm in view of the fact that both the absorption lossand the resistance are reduced.

[0411] By using the distributed Bragg reflector of the embodiment ofFIG. 22 for the reflector in the surface-emission laser diode, it ispossible to obtain a surface-emission laser diode having a superiorcharacteristic. As noted before, it has been difficult to realize thethickness of 30-50 nm for the heterospike buffer layer in theconventional distributed Bragg reflector tuned to the 0.85 μmwavelength. The heterospike buffer layer of this thickness becamepossible for the first time in the distributed Bragg reflector tuned tothe wavelength of 1.1 μm.

[0412] In the distributed Bragg reflector of the present invention, itis also possible to set the difference of Al content between the lowrefractive index layer 18 a and the high refractive index layer 18 b ofthe distributed Bragg reflector to be less than 0.8 by constituting thelayers 18 a and 18 b by any of AlAs, GaAs and AlGaAs mixed crystal.

[0413] When the difference of Al content between the high refractiveindex layer 18 b and the low refractive index layer 18 a is less than0.8 in the distributed Bragg reflector constituted of a semiconductormaterial of the AlGaAs system and tuned to the design wavelength of 1.1μm or longer, it is possible to reduce the electric resistance whilemaintaining a high reflectance.

[0414] In a mixed crystal of AlGaAs, it should be noted that the valenceband energy decreases monotonously with the increase of Al contenttherein, and the band discontinuity of the valence band at the interfaceto a GaAs crystal increases with increasing Al content in the AlGaAsmixed crystal. Thereby, there is formed a large potential barrier at thehetero interface, and this potential barrier has been the cause of thehigh resistance of the distributed Bragg reflector. Further, it shouldbe noted that the decrease of the valence band energy is generallyproportional to the Al content, and the band discontinuity between thesemiconductor layers having different Al compositions correspond to theAl compositional difference.

[0415]FIGS. 23 and 24 show the resistivity of the p-type distributedBragg reflector tuned to the design wavelength of 1.3 μm and including 4pairs of high refractive index layer 18 b and low refractive index layer18 a for various thicknesses of the heterospike buffer layer. In FIGS.23 and 24, it should be noted that the distributed Bragg reflector usesGaAs for the high refractive index layer 18 b and AlAs,Al_(0.8)Ga_(0.2)As and Al_(0.6)Ga_(0.4)As have been used for the lowrefractive index layer Further, each of the layers has been formed tohave a thickness tuned to ¼ the design wavelength while taking intoconsideration of the refractive index of the respective layers. Further,the doping density is set to 5×10¹⁷ cm⁻³ throughout the layers in FIG.23 and 1×10¹⁸ cm⁻³ throughout the layers in FIG. 24. It should be notedthat the doping density of 1×10¹⁸ cm⁻³ is used commonly in the doping ofthe conventional p-type distributed Bragg reflector.

[0416] From FIGS. 23 and 24, it can be seen that the resistivity of thedistributed Bragg reflector increases with increasing Al content in theAlGaAs layer and decreasing thickness of the heterospike buffer layer,and that it is necessary to provide a thick heterospike buffer layer inorder to reduce the resistivity of the distributed Bragg reflector tothe bulk resistivity level. In the case of the distributed Braggreflector that uses Al_(0.6)Ga_(0.4)As and GaAs as the low refractiveindex layer 18 a and the high refractive index layer 18 b, for example,there appears a band discontinuity of about 300 meV, while the banddiscontinuity increases to about 400 meV when Al_(0.8)Ga_(0.2)As is usedfor the low refractive index layer 18 a and GaAs is used for the highrefractive index layer 18 b.

[0417] From the results of FIGS. 23 and 24, it is concluded that thethickness of 20 nm or more is preferable for the heterospike bufferlayer 18 c in order to reduce the resistivity of a distributed Braggreflector formed of AlAs layers and GaAs layers in order to reduce theresistivity of the distributed Bragg reflector effectively, althoughthis would also depend on the doping density. Thus, in the case the banddiscontinuity is 400 meV or less, in other words the Al compositionaldifference is less than 0.8, it can be seen that the resistivity of theheterospike buffer layer can be reduced effectively by increasing thethickness of the heterospike buffer layer to 20 nm or more.

[0418] Actually, the increase of resistance of the distributed Braggreflector caused by the band discontinuity depends not only on thebarrier height and barrier thickness but also on the effective mass ofholes used for the carriers. However, there is no large difference inthe effective mass for heavy holes between AlGaAs, AlGaInP and GaInAsP,and the band discontinuity can be regarded as the index of theheterointerface resistance. Thus, in the case the valence banddiscontinuity is less than 400 mev, and hence the Al compositionaldifference is less than 0.8, it is possible to reduce the resistance ofthe distributed Bragg reflector by using the heterospike buffer layerhaving a thickness of 20 nm or more. With regard to the upper limit ofthe heterospike buffer layer 18 c, the thickness of the heterospikebuffer layer 18 c should be chosen, in view of the tuned wavelength ofthe distributed Bragg reflector, such that no remarkable decrease ofreflectance occurs. By doing so, a distributed Bragg reflector havingexcellent electrical and optical characteristics is obtained.

[0419] In the distributed Bragg reflector having a structure shown inFIG. 3, it is possible to use any of AlAs, GaAs and AlGaAs mixed crystalfor the low refractive index buffer layer 18 a and the high refractiveindex layer 18 b such that the Al compositional difference between thelayers 18 a and 18 b is 0.8 or more.

[0420] In this embodiment, it is also possible to reduce the resistanceof the distributed Bragg reflector effectively while maintaining a highreflectance.

[0421] When AlAs and GaAs are combined for the low refractive indexlayer 18 a and the high refractive index layer 18 b, it should be notedthat there appears a band discontinuity of about 500 meV in the valenceband, and thus, it is necessary to provide a thick heterospike bufferlayer for reducing the resistivity of the distributed Bragg reflector,although the thickness of the heterospike buffer layer may depend on thedoping density. In such a case, it is concluded, in view of the resultsof FIGS. 23 and 24, that the heterospike buffer layer 18 c has athickness of 30 nm or more. Thus, from the view point of banddiscontinuity of the semiconductor materials constituting thedistributed Bragg reflector, it becomes possible to reduce theresistance of the distributed Bragg reflector by setting the thicknessof the heterospike buffer layer to be 30 nm or more in the case thereexists a band discontinuity is 400 meV or more.

[0422] Thus, there holds a relationship explained with reference toFIGS. 23 and 24 between the Al compositional difference and the valenceband discontinuity, and the valence band discontinuity of 400 mevcorresponds to the Al compositional difference of 0.8 or more. Thus, inthe case the Al compositional difference is 0.8 or more, the thicknessof 30 nm or more is needed for the heterospike buffer layer 18 c so asto reduce the resistance effectively. By providing such a heterospikebuffer layer as such, it is possible to reduce the resistance of thedistributed Bragg reflector effectively.

[0423] With regard to the upper limit of the heterospike buffer layer,the thickness of the heterospike buffer layer 18 c is chosen in view ofthe tuned wavelength of the distributed Bragg reflector such that thereoccurs little decrease of reflectance. Thereby, it becomes possible torealize a distributed Bragg reflector having excellent characteristicsboth in terms of electric properties and optical properties.

[0424] In the distributed Bragg reflector of the present invention tunedto the design wavelength of 1.1 μm, it is also possible to set thethickness of the heterospike buffer layer 18 c with respect to the tunedwavelength λ [nm] to be equal to or smaller than (50λ−15) [nm]. In sucha case, too, it is possible to reduce the resistance of the distributedBragg reflector while maintaining high reflectance.

[0425]FIG. 25 shows the relationship between the thickness of theheterospike buffer layer and reflectance of the distributed Braggreflector tuned to the wavelength of 1.1-1.7 μm. It should be noted thatthe distributed Bragg reflector has a structure explained with referenceto FIG. 3 and uses GaAs for the high refractive index layer 18 b andAlAs for the low refractive index layer 18 a. The layers 18 a and 18 bare repeated with the number determined such that the reflectanceexceeds 99.9% at each of the tuned wavelengths. Thus, in the case of thereflector is tuned to 0.88 μm, the layers 18 a and 18 b are repeated 18times, while in the case of the reflector is tuned to 1.1 μm, the layers18 a and 18 b are repeated 22 times. Further, in the case the reflectoris tuned 1.3 micron, the layers 18 a and 18 b are repeated 23 timeswhile in the case the reflector is tuned to 1.5 μm, the layers 18 a and18 b are repeated 23 times. Further, in the case the reflector is tunedto 1.7 μm, the layers 18 a and 18 b are tuned to 24 times.

[0426]FIG. 26 shows a change rate of the resistivity of the distributedBragg reflector shown in FIG. 25 (|dR/dt|) with the thickness of theheterospike buffer layer.

[0427] From FIG. 25, it can be seen that reflectance of the reflectordecreases with increasing thickness of the heterospike buffer layer.Further, FIG. 26 shows that the decrease of the reflectance increasessuddenly at a certain thickness of the heterospike buffer layer. For thesake of facilitating understanding of this situation, FIG. 26 showslines drawn as a tangential for each of the curves as the referencereflectance change rate. For example, in the distributed Bragg reflectortuned to 1.3 μm, it can be seen from FIG. 26 that the change rate of thereflectance increases sharply when the thickness of the heterospikebuffer layer 18 c has exceeded 50 nm. In correspondence thereto, thereoccurs a sharp drop in the reflectance of the distributed Braggreflector. Thus, in the surface-emission laser diode having such adistributed Bragg reflector for the reflector, the threshold current oflaser oscillation increases sharply at the foregoing thickness of theheterospike buffer layer 18 c. Further, it should be noted that thethickness of the heterospike buffer layer 18 c corresponding to such asharp change of reflectance changes depending on the design wavelengthof the laser diode. Thus, longer the design wavelength of the laserdiode, the thickness of the semiconductor layers constituting thereflector is increased, and the effect of the heterospike buffer layeris reduced.

[0428] Thus, the thickness of the heterospike buffer layer correspondingto such a sudden increase of change rate of the reflectance changeddepending on the design wavelength, while the threshold of the changerate corresponding to the onset of the sudden growth does not changewith the design wavelength and maintains the value of about 0.9 as canbe seen from FIG. 26.

[0429] Table 2 below summarizes the threshold thickness for eachwavelength shown in FIG. 25. TABLE 2 design wavelength 1.1 μm 1.3 μm 1.5μm 1.7 μm threshold thickness 40 nm 50 nm 60 nm 70 nm

[0430] From Table 2, it can be seen that the design wavelength and thethreshold thickness are in a generally linear relationship and thereholds a relationship between the threshold thickness t [nm] and thedesign wavelength λ [nm] of the distributed Bragg reflector as follows:

t=50λ−15.  (1)

[0431] Thus, by providing the heterospike buffer layer 18 c in thedistributed Bragg reflector tuned to the wavelength of 1.1 μm or morewith the thickness not exceeding the thickness t given by Equation (1),it is possible to realize a low-resistance distributed Bragg reflectorhaving a high reflectance.

[0432] In the example above, it was assumed that the heterospike bufferlayer has a linear compositional graduation profile. However, it is alsopossible to use non-linear profile In this case, too, similar resultsand effects are achieved.

[0433] In the present invention, it is possible to increase thethickness of the heterospike buffer layer to be 20 nm or more in thedistributed Bragg reflector having a thickness of 1.1 μm. In this case,too, it is possible to reduce the resistance of the mirror whilemaintaining a high reflectance.

[0434] As explained before, there holds a relationship of Equation (1)between the thickness of the heterospike buffer layer and the designwavelength of the distributed Bragg reflector that can maintain a highreflectance.

[0435] With regard to the electric properties of the distributed Braggreflector, it is possible to reduce the effect of the hetero interfaceby increasing the thickness of the heterospike buffer layer as notedbefore, and a distributed Bragg reflector having a reduced resistance isobtained. Further, the effect of the resistance reduction by theheterospike buffer layer id determined by the materials used for thedistributed Bragg reflector and the doping density and further by thecompositional profile. Essentially, it does not depend on the reflectionwavelength. Thus, it is concluded that there exists a lower limit in thethickness of the heterospike buffer layer in which the resistance of thedistributed Bragg reflector is decreased sufficiently. In order toprovide a distributed Bragg reflector having a sufficiently lowresistance, it is necessary to provide the heterospike buffer layer witha thickness exceeding a predetermined thickness.

[0436] In the example of the distributed Bragg reflector of FIG. 24 inwhich the semiconductor layers are doped uniformly to the doping densityof 1×10¹⁸ cm⁻³, it can be seen that the resistivity of the distributedBragg reflector increases in terms of orders when the thickness of thedistributed Bragg reflector is les than 20 nm. When the thickness of theheterospike buffer layer has exceeded the value of 20 nm, on the otherhand, it is preferable to use the thickness of 20 nm or more for theheterospike buffer layer, provided that the semiconductor layersconstituting the distributed Bragg reflector has the foregoing doingdensity.

[0437] From the description noted above, it is concludes that adistributed Bragg reflector having a sufficiently low resistance andsimultaneously maintaining a high optical reflectance is obtained bychoosing the thickness t [nm] of the heterospike buffer layer withregard to the design wavelength λ of the distributed Bragg reflector soas to satisfy the relationship 20≦t≦50λ−15.

[0438] In the distributed Bragg reflector of this embodiment, it is alsopossible to set the thickness of the heterospike buffer layer to be 30nm or more. Even in such a case, it is possible to decrease theresistance of the reflector effectively while maintaining a highreflectance at the tuned wavelength or design wavelength of 1 μm orlonger.

[0439] In semiconductor materials, there is a tendency that opticalabsorption increases also for the photons having energy smaller than thebandgap with increase of free carriers. Further, in the case of thep-type semiconductor materials, there arises conspicuous opticalabsorption caused as a result of the valence band to valence bandabsorption with the increase holes acting as the carriers. As theproblem of the valence band to valence band absorption becomesconspicuous with increasing optical wavelength, this problem becomes aserious problem in the distributed Bragg reflector having a longwavelength of 1.1 μm or more. It should be noted that such opticalabsorption becomes the cause of decreasing the reflectance of thedistributed Bragg reflector. In the case of the laser diode using such areflector, there arises a problem of increase of threshold currentcaused by optical absorption and decrease of efficiency. Thus, from theview point of decreasing the optical absorption, it is preferable thatthe doping density of the semiconductor layers is set as small aspossible. However, with the decrease of the doping density, there arisesa problem of increase of thickness of the heterointerface, and theeffect of the interface potential is increased, thus leasing to theproblem of increase of resistance of the distributed Bragg reflector.

[0440] Thus, in the case of the semiconductor distributed Braggreflector having the reduced doping density, it is necessary to use athicker heterospike buffer layer for reducing the resistivity. In thecase of doping the semiconductor layers constituting the distributedBragg reflector to the doping density of 5×10¹⁷ cm⁻³, it can be seenfrom FIG. 23 that the resistivity is reduced to the level of the bulkmaterial when the thickness of the heterospike buffer layer is increasedto 30 nm or more.

[0441] With regard to the doping density and profile, there are numerouscombinations, while there arises a similar tendency when the dopingdensity of one of the semiconductor layers is less than 1×10¹⁸ cm⁻³.This is because the potential barrier formed at the hetero interface hasthe height and thickness depending on the doping density of thesemiconductor layer adjacent to the hetero interface. Lower the dopingdensity, as in the case of the doping density having a value of 1×10¹⁸cm⁻³, larger the influence of the hetero interface. Further, theelectric property of the distributed Bragg reflector is mainlydetermined by the hetero interface where the doping density is small.Thus, the present invention is effective also in the case in which atleast one of the semiconductor layers constituting the distributed Braggreflector has a doping density of less than 1×10¹⁸ cm⁻³.

[0442] It should be noted that a similar tendency appears also in thecase of the distributed Bragg reflector doped to the order of 1×10¹⁷cm⁻³. Of course, it is possible to reduce the doping density further,provided that the thickness of the heterospike buffer layer is increasedto 30 nm or more within the upper limit noted before.

[0443] Thus, according to the present embodiment, it is possible toobtain a distributed Bragg reflector having a sufficiently lowresistance and high optical reflectance by choosing the thickness t [nm]of the heterospike buffer layer with regard to the design wavelength λof the laser diode so as to fall in the range of 30≦t≦50λ−15.

[0444] In the conventional laser diode operable at the wavelength bandof 0.85 μm, there has been a study to provide a heterospike buffer layeras noted above. On the other hand, such a heterospike buffer layer ismost effectively used in the long-wavelength surface-emission laserdiode operable at the wavelength of 1.1-1.7 μm. In the 1.1-1.7 μm band,for example, it is possible to set the thickness of the material layerconstituting the heterospike buffer layer about twice the thickness forthe case of the 0.85 μm band, in order to obtain the same reflectance(99.5% or more, for example). Thereby, the resistance of thesemiconductor Bragg reflector is reduced, and the operational voltage,oscillation threshold current, and the like, are likewise reduced.Thereby, advantageous features such as suppression of heating, stablelaser oscillation and low energy drive is obtained for the laser diode.

[0445] Thus, the provision of such a heterospike buffer layer to thesemiconductor Bragg reflector according to the present invention isdeemed an advantageous improvement especially in the case of thelong-wavelength surface-emission laser diode operable in the laseroscillation wavelength of 1.1-1.7 μm.

[0446] For example, in the case of the surface-emission laser diodeoperable at the wavelength band of 1.3 μm and having a semiconductorBragg reflector in which a low refractive index layer of AlxGal-xAs(x=1.0) and a high refractive index layer of AlyGa1-yAs (y=0) arestacked for 20 periods, a reflectance of 99.7% or less is obtained,provided that the thickness of the heterospike buffer layer AlzGa1-zAs(0≦y<z<x≦1) is set to 30 nm. Further, a reflectance of 99.5% or more isobtained when the heterospike buffer layer has a thickness of 53 nm.Thus, a film thickness control of ±2% is sufficient when the distributedBragg reflector is designed to have a reflectance of 99.5% or more. Inthe experiments, distributed Bragg reflectors were produced with thethickness of 10 nm, 20 nm and 30 nm for the heterospike buffer layer. Itturned out that a reflectance sufficient for practical use was achievedin any of these experiments. In this way, a 1.3 μm band surface-emissionlaser diode having a reduced resistance for the semiconductor Braggreflector was realized and laser oscillation was achieved. The remainingstructural feature of the laser diode thus produced will be explainedlater.

[0447] In a multilayer reflector, there exists a band of highreflectance in the designed wavelength band (assuming that complete filmthickness control is achieved). This is called high reflectance band,wherein the high reflection band may include also the wavelength band inwhich a reflectance exceeding a target value is achieved for a targetwavelength. In the high reflectance band, the reflectance becomeslargest at the designed wavelength, while the reflectance falls off butonly slightly as the deviates from the designed wavelength. When thedeviation exceeds a certain limit, the reflectance falls off sharply.

[0448] Thus, in a multilayer reflector, it is necessary to control thethickness of the multilayer reflector completely with atomic layer levelso that a reflectance exceeding the necessary reflectance is thatachieved at the target wavelength. In practice, a deviation of about ±1%is inevitable for the film thickness, and thus, it is a common placethat the target wavelength and the wavelength in which the reflectanceis maximized are different In the case the target wavelength is 1.3 μm,for example, the wavelength providing the maximum reflectance isdeviated by 13 nm when there it an error of ±1% in the film thicknesscontrol. Therefore, it is desirable that this high reflectance band hasa wide bandwidth. Here the high reflectance band is defined as thewavelength band in which a reflectance exceeding a necessary reflectanceis obtained for a target wavelength.

[0449] Thus, in the long-wavelength surface-emission laser diodeoscillating at the wavelength of 1.1-1.7 μm, it is possible to reducethe resistance value of the semiconductor Bragg reflector whilemaintaining high reflectance, by optimizing the constitution of thereflector. Thereby, the operating voltage, oscillation thresholdcurrent, and the like, of the laser diode are successfully reduced, andheat generation is suppressed effectively. As a result, stable laseroscillation is realized and low energy driving of the laser diodebecomes possible.

[0450] Once again FIG. 1 is referred to.

[0451] It can be seen that a p-type GaAs layer 19 having a compositionrepresented as AlxGal-xAs (x=0) is provided on the upper semiconductorBragg reflector 18 as a contact layer (p-contact layer), so as toachieve a contact with the p-side electrode 20.

[0452] In the constitution of FIG. 1, it should be noted that the Incontent x of the quantum well active layer is set to 39% (Ga0.61In0.39As). Further, the thickness of the quantum well active layeris set to 7 nm. The quantum well active layer thus formed accumulates acompressional strain of about 2.8% with respect to the GaAs substrate.

[0453] In the surface-emission laser diode of FIG. 1, the deposition ofthe semiconductor layers is conducted by an MOCVD process. In this case,no lattice-relaxation phenomenon was observed. Each of the semiconductorlayers of the laser diode may be formed by using TMA (trimethylaluminum), TMG (trimethyl gallium), TMI (trimethyl indium), AsH₃(arsine), PH₃ (phosphine) as source materials, together with a carriergas of H₂. In the case of growing the active layer (the quantum wellactive layer) in the form of highly strained layer as in the case of thedevice of FIG. 1, it is preferable to use a low temperature growthprocess that realizes a non-equilibrium growth. In the present case, theGaInAs layer 15 a (quantum well active layer) is grown at 550° C. Itshould be noted that the MOCVD process used herein is characterized byhigh degree of supersaturation and is suited for the crystal growth ofhighly strained active layer. Further, the MOCVD process does notrequire high vacuum environment as in the case of MBE process. Further,the MOCVD process is suitable for mass production, as the processcontrol is achieved by merely controlling the supply rate or supplyduration of the source gases.

[0454] In the illustrated laser diode, there is formed a high resistanceregion 15F outside the current path by means of ion implantation ofprotons (H+) as a current confinement structure.

[0455] In the constitution of FIG. 1, it should be noted that there isformed a p-side electrode 20 on the p-type contact layer formed in turnon the uppermost part of upper part reflector 18 and constituting a partthereof, except for an optical exit part 20A. Also there is formed ann-side electrode 21 at the rear surface of the substrate.

[0456] In the present embodiment, it should be noted that the activeregion including the upper part and lower part spacer layers 14 and 16in addition to the multiple quantum well active layer 15 and forming theresonator between the upper and lower reflectors 12 and 18, in which thecareer recombination is caused upon injection of carriers, is formed ofa material not containing Al (the proportion to the group III element(s)is 1% or more). Furthermore the low refractive index layers constitutingthe lower part reflector 12 and also the upper part reflector 18 closestto the active layer is formed of the non-optical recombinationelimination layers 13 and 17 each having a composition ofGaxIn1-xPyAs1-y (0<x≦1, 0<y≦1), in which the compositional parameters xand y are chosen appropriately. More specifically, GaInP or GaInPAs orGaPAs is used for the non-optical recombination elimination layer. Thematerial constituting the non-optical recombination elimination layerthus has a composition of GaxIn1-xPyAs1-y (0<x≦1, 0<y≦1), wherein thenon-optical recombination layer may be added further with a trace amountof element(s) other than Al.

[0457] In such a structure, the careers are confined between the lowrefractive index layer of the upper part reflector and the lowrefractive index layer of the lower part reflector that are locatednearest to the active layer and formed of a wide gap material. In such astructure, there occurs non-optical recombination of carriers even whenthe active region is formed of the layer not containing Al (theproportion of Al with regard to group III elements is 1% or less) uponinjection of the careers, as there occurs non-optical recombination ofcarriers at the interface between the active layer and the lowrefractive index layer of the upper or lower reflector, as long as theforegoing low refractive index layer (wide gap layer) contains Al. Whenthe non-optical recombination of carriers takes place, the efficiency ofoptical emission is deteriorated. Therefore it is desirable to form notonly the active region but also the low refractive index layersadjoining thereto by a material not containing Al.

[0458] Further, the non-optical recombination elimination layer havingthe primary composition of GaxIn1-xPyAs1-y (0<x≦1, 0<y≦1) has a latticeconstant smaller than the lattice constant of the GaAs substrate. Thus,the layer accumulates a tensile strain.

[0459] In an epitaxial growth process, the growth is made whilereflecting the information of the foundation layer on which the growthis made. Thus, when there are defects on surface of the foundationlayer, the same defects crawl up to the growth layer. On the other hand,it is known that such a crawling up of the defects can be suppressed byinterposing a strained layer on the path of the defects.

[0460] In the case an active layer accumulates therein a strain, theresometimes occurs a problem in that growth of the active layer withdesired thickness is difficult because of the reduced criticalthickness. Particularly, there arises a problem in that the film growthdoes not take place at all even if a low temperature growth process ornon-equilibrium growth process is employed, due to the existence ofdefects. This problem occurs particularly when the active layeraccumulates a compressive strain of 2% or more, or when growing theactive layer beyond the critical thickness thereof.

[0461] On the other hand, when there exists a strained layer adjacent tothe active layer, such a crawling up of the defects is intercepted andthe efficiency of optical emission is improved. Further, it becomespossible to grow the active layer even in the case the active layeraccumulates a compressive strain of 2% or more. Further, it is possibleto grow the strained layer beyond the critical thickness.

[0462] It should be noted that the GaxIn1-xPyAs1-y (0<x≦1, 0<y≦1) layers13 and 17 adjoin the active region and function so as to confine thecareers into the active region, On the other hand, the GaxIn1-xPyAs1-y(0’x≦1, 0<y≦1) layers 13 and 17 also have the feature in that thebandgap energy can be increased by decreasing the lattice constant. Inthe case of GaxIn1-xP (y=1), for example, there occurs an increase ofthe lattice constant when the compositional parameter x has increasedand the film composition has approached the composition GaP. Associatedtherewith, there occurs an increase of bandgap. It should be noted thatthe bandgap energy Eg is given as Eg (┌)=1.351+0.643x+0.786x² in thecase of direct transition and Eg (X)=2.24+0.02x in the case of indirecttransition. Therefore, the hetero barrier height between the activeregion and the GaxIn-xPyAs1-y (0<x≦1, 0<y≦1) layer 13 or 17 is increasedand the degree of career confinement is improved. Thereby, the thresholdcurrent is reduced and the temperature characteristics are improved.

[0463] Furthermore, it should be noted that the non-opticalrecombination elimination layer 13 or 17 having the compositionGaxIn1-xPyAs1-y (0<x≦1, 0<y≦1) has a lattice constant larger than thelattice constant of the GaAs substrate. Also the lattice constant of theactive layer is larger than the lattice constant of the GaxIn1-xPyAs1-y(0<x≦1, 0<y≦1) layer 13 or 17. Thus, these layers accumulatecompressional strain therein. As the direction of the strain accumulatedin the GaxIn1-xPyAs1-y (0<x≦1, 0<y≦1) layer is the same as the directionof the strain accumulated in the active layer, the substantial amount ofthe compressional strain that the active layer senses is reduced. Largerthe strain, the influence of external factor is also large. Thus, theconstruction of the present invention is especially effective in thecase that the active layer accumulates a large compressional strain of2% or more, or the critical film thickness has been exceeded.

[0464] It is preferable to form a surface-emission laser diode of the1.3 μm band on a GaAs substrate. Further, there are many cases in whicha semiconductor multilayer reflector is used for the resonator. In sucha case, it is necessary to grow 50 to 80 semiconductor layers with atotal thickness of 5-8 μm. (In the case of an edge-emission type laserdiode, on the other hand, the total thickness before growth of theactive layer is about 2 μm, and itg is sufficient to grow just aboutthree layers.)

[0465] Thus, even if a high quality GaAs substrate is used, increase ofdefect density of the surface on which the growth of the active layer ismade, in the sate immediately before the growth of the active layergrowth is inevitable. Once formed, the defects thus formed crawl up inthe direction of the crystal growth. Further, there may be additionaldefect formation at the hetero surface. On the other hand, measures suchas provision of strain layer before the growth of the active layer orreduction of the substantial strain which the active layer senses, areeffective for reducing the influence of the defects on the surface onwhich the growth of the active layer is made in the state immediatelybefore the growth of the active layer.

[0466] In this embodiment, Al is expelled from the active layer andfurther from the interface region between the active region and thereflector. Thus, the problem of non-optical recombination of carriersoriginating from the crystal defects and caused by Al at the time ofcareer injection is successfully removed.

[0467] As noted before, while it is preferable to provide thenon-optical recombination elimination layer not containing Al at theinterface of both of the reflectors 12 and 18, beneficial effect can beobtained also in the case the non-optical recombination eliminationlayer is provided to only one of the reflectors. In the illustratedexample, both of the upper and lower reflectors 12 and 18 are formed ofthe semiconductor Bragg reflector. However, it is possible to form oneof the reflectors by the semiconductor Bragg reflector and form theother reflector by a multilayer dielectric reflector.

[0468] In the foregoing example, only the low refractive index layercloset to the active layer forms the GaxIn1-xPyAs1-y (0<x≦1, 0<y≦1)non-optical recombination elimination layer 13 or 17 in any of thereflectors 12 and 18. However, it is possible to form the non-opticalrecombination elimination layer 13 or 17 by a plurality ofGaxIn1-xPyAs1-y (0<x≦1, 0<y≦1) layers.

[0469] In this embodiment, the above concept is applied particularly tothe lower reflector 12 located between the GaAs substrate and the activelayer, and the problem of crawling up of crystal defects originatingfrom Al at the time of the growth of the active layer and the adversaryinfluence thereof, are suppressed successfully. As a result, the activelayer can be grown with high quality, and a highly reliablesurface-emission laser diode oscillating with high efficiency andsufficient for practical use is obtained. In the present embodiment, theAl-free, GaxIn1-xPyAs1-y (0<x≦1, 0<y≦1) layer is used only for the lowrefractive index layer located closest to the active region in thesemiconductor Bragg reflector. Therefore, it is possible to achieve theforegoing effect without increasing the number of stacks of thereflector.

[0470] The surface-emission laser diode thus produced oscillatedsuccessfully at the wavelength of about 1.2 μm. While the wavelength ofGaInAs formed on a GaAs substrate increases with increase of In contenttherein, such an increase of the In content is accompanied with anincrease of strain. Thus, it has been thought that the wavelength of 1.1μm would be the limit of the increase of laser oscillation wavelength inthe laser diode that uses GaInAs. See IEEE Photonics. Technol. Lett.Vol.9 (1997) pp.1319-1321.

[0471] In the present invention, the inventor could successfully achievethe laser oscillation at 1.2 μm by using highly non-equilibrium growthprocesses conducted at low temperature of 600° C., or less. By usingsuch a process, it became possible to achieve a coherent growth of thehighly strained GaInAs quantum well active layer with large thicknessnot possible before Meanwhile, this wavelength is transparent withrespect to a Si semiconductor substrate. Thus, the laser diode of thepresent invention can be used to construct a circuit chip that usesoptical transmission through the Si substrate, by integrating anelectronic device and an optical device commonly on a Si substrate.

[0472] From the explanation noted above, it was discovered that along-wavelength surface emission laser diode can be constructedsuccessfully on a GaAs substrate by using a highly strained GaInAs layercontaining a large amount of In and hence a large amount of compressivestrain for the active layer.

[0473] As noted before, such a surface-emission laser diode can beformed by using an MOCVD process. However, it is also possible to use anMBE process or other growth process for this purpose. In the embodimentdescribed heretofore, a triple quantum well structure (TQW) was used forthe active layer However, it is also possible to use a structure havingdifferent number of quantum well layers (SQW, MQW).

[0474] Further, it should be noted that the laser diode may also have adifferent structure. Further, the resonator length is not limited toAbut may have a length of integer multiple of λ/2 or preferably λ.

[0475] In the embodiment described heretofore the laser diode wasconstructed on the GaAs substrate. However, it is also possible to usean InP substrate. Further, the period of repetition of in the reflectorsmay be changed.

[0476] In the embodiment described heretofore, the active layer ofGaxIn1-xAs (GaInAs active layer) containing Ga, In and As as the majorelement is used in this example. On the other hand, it is possible toadd N to the active layer of the laser diode. In this case, the activelayer contains Ga, In, N and As as major elements (GaInNAs activelayer), and the laser diode can oscillate at further longer wavelengths.

[0477] By changing the composition of the GaInNAs active layer, it ispossible to achieve laser oscillation at any of the 1.3 μm band and 1.55μm band. By choosing the composition of the active layer appropriately,it is also possible to realize a surface-emission laser diode laseroscillating at further longer wavelengths such as 1.7 μm.

[0478] It is also construct a surface-emission laser diode operable atthe wavelength of 1.3 μm band on a GaAs substrate by using GaAsSb forthe active layer.

[0479] Conventionally, there has been no material suitable for realizinga laser diode operable at the wavelength of 1.1-1.7 μm. By using ahighly strained layer of GaInAs, GaInNAs or GaAsSb for the active layer,and by using a non-optical recombination elimination layer, the presentinvention successfully realized a highly efficient surface-emissionlaser diode operable in the long wavelength region of a/the 1.1-1.7 μmband.

[0480] [Second Embodiment]

[0481] Next, the constitution of another long-wavelengthsurface-emission laser diode applicable to an opticaltransmission/reception system of this invention as a light-emittingdevice that is will be described by using FIG. 27. In the drawing, thoseparts corresponding to the parts described previously are designated bythe same reference numerals and the description thereof will be omitted.

[0482]FIG. 27 is referred to.

[0483] Similarly to the previous embodiment of FIG. 1, this embodimentalso uses an n-type GaAs substrate 11 having a surface orientation of(100).

[0484] the GaAs substrate 11, it can be seen that the n-typesemiconductor Bragg reflector (Al_(0.9)Ga_(0.1)As/GaAs lower partreflectors) 12 is formed by alternately depositing the n-type AlGaAshaving a composition of AlxGa1-xAs (x=0.9) and an n-type GaAs having acomposition of AlxGa1-xAs (x=0) alternately for 35 periods, each with athickness of ¼ times the oscillation wavelength λ in each medium(thickness of λ/4). Further, the n-type InGaP layer 13 is formed on thelower Bragg reflector 12 with a composition of GaxIn1-xPyAs1-y (x=0.5,y=1) and a thickness of λ/4. Thereby, the n-type GaxIn1-xPyAs1-y (x=0.5,y=1) layer 13 forms one of the low refractive index layers constitutinga part of the lower part reflector 12.

[0485] On the InGaP layer 13, the multiple quantum well active layer 15is formed by stacking the undoped lower GaAs spacer layer 14 and furtherthe GaxIn1-xNyAs1-y quantum well active layer 15 a three times on thespacer layer 14, with the GaAs barrier layer 15 b of 15 nm thicknessinterposed therebetween to form a triple quantum wells (TQW) structure.Further, the undoped upper GaAs spacer layer 16 is formed on themultiple quantum well active layer 15 thus formed, wherein the lowerspacer layer 14, the multiple quantum well active layer 15 and the upperGaAs spacer layer 16 constitute together a resonator 15R having athickness of one wavelength (λ) of the oscillation wavelength in themedium. The resonator 15R constitutes the active region of thesurface-emission laser diode.

[0486] Further, the p-type semiconductor Bragg reflector (the upper partreflector) 18 is formed on the resonator 15R.

[0487] It should be noted that the upper part reflector 18 includes alow refractive index layer having a thickness of 3λ/4 formed of an AlAslayer 18 ₁ used for a selective oxidizing layer, wherein the foregoinglow refractive index layer is sandwiched by the GaInP layer 17 and anAlGaAs layer. The GaInP layer 17 is doped with C has a compositionrepresented as GaxIn1-xPyAs1-y (x=0.5, y=1), wherein the GaInP layer 17is formed to have a thickness of λ/4-15 nm. On the other hand, the AlAslayer constituting the selective oxidizing layer is doped with C and hasa composition represented as AlzGal-zAs (z=1) and a thickness of 30 nm.Further, the foregoing AlGaAs layer is a C-doped AlGaAs layer having acomposition represented by AlxGa1-xAs (x=0.9) and a thickness of 2λ/4-15nm.

[0488] On the foregoing low refractive index layer, a GaAs layer havinga thickness of λ/4 is formed for one period, and a p-type AlGaAs layer,doped with C and having a composition of expressed with AlxGa1-xAs(x=0.9) and a p-type GaAs layer doped with C and having a compositionrepresented by AlxGa1-xAs (x=0), are stacked 22 periods each with athickness of ¼ times the laser oscillation wavelength in each medium.Thereby a periodic stack structure constituting the essential part ofthe upper part reflector 18 is formed.

[0489] In this embodiment, too, the heterospike buffer layer 18 c ofintermediate refractive index is provided in the semiconductor Braggreflector 18 by an AlzGa1-zAs (0≦y<z<x≦1) layer between the lowrefractive index layer 18 a and the high refractive index layer 18 b asalready explained with reference to FIG. 2. In FIG. 27, illustration ofthe heterospike buffer layer 18 c will be omitted for the sake ofsimplicity.

[0490] In this embodiment, the p-type GaAs layer having the compositionof AlxGallxAs (x=0) and constituting the uppermost part of thesemiconductor Bragg reflector 18 functions as a contact layer (p-contactlayer) that secures electrical contact to the electrode.

[0491] In the surface-emission laser diode of this embodiment, the Incontent x of the quantum well active layer 15 a is set to 37% and the N(nitrogen) content is set to 0.5%. The thickness of the quantum wellactive layer 15 a is set to 7 nm.

[0492] In this embodiment, the growth of the semiconductor layersconstituting the said surface-emission laser diode was conducting byusing an MOCVD process. More specifically, TMA (trimethyl aluminum), TMG(trimethyl gallium), TMI (trimethyl indium), AsH3 (arsine) and PH3(phosphine) are used respectively as the source materials of Al, Ga, In,As and P. Further, DMHy (dimethyl hydrazine) was used as the sourcematerial of nitrogen. DMHy decomposes at a low temperature and is suitedfor a low temperature growth process conducted at 600° C. or less.Particularly, it is suited to grow a quantum well layer of large strain,which needs a low temperature growth process. In the MOCVD process, H₂is used for the carrier gas. Further, the growth of the GaInNAs layer(quantum well active layer) was conducted at 540° C.

[0493] It should be noted that MOCVD process is characterized by largedegree of supersaturation and is suited for the crystal growth of amaterial that contains N and simultaneously other group V elements,Further, the MOCVD process does not require the high vacuum environment,contrary to the MBE process. Further, it is suitable for massproduction, as it is only sufficient to control the supply rate andsupply time of the source gases.

[0494] In this embodiment, a predetermined part of the stacked structurethus formed is etched until it reaches the p-type GaxIn1-xPyAs1-y(x=0.5, y=1) layer 17. Thereby, there is formed a mesa structure thatexposes the p-AlzGa1-zAs (z=1) selective oxidizing layer 181 at thesidewall thereof. Furthermore, said AlzGa1-zAs (z=1) layer 18 ₁ thusexposed is oxidized from the mesa sidewall by water vapor, and there isformed a current confinement layer 182 having a composition representedas AlxOy.

[0495] Finally, the part removed with the mesa etching processpreviously is filled by polyimide to form a planarized structure, andthe polyimide film covering the upper part reflector is removed. Withthis, there is formed a polyimide region 183. Furthermore the p-sideelectrode 20 is formed on the p-type contact layer except for theoptical exit part, and an n-side electrode 21 is formed on the rear sideof the GaAs substrate 11.

[0496] In this embodiment, the GaxIn1-xPyAs1-y (0<x≦1, 0<y≦1) layer 17is inserted below the selective oxidizing layer 18, as a part of theupper part reflector 18. In the case a wet etching process by using asulfuric acid etchant is employed in the formation of the mesastructure, the etching stops spontaneously at the GaxIn1-xPyAs1-y(0<x≦1, 0<y≦1) layer 17, as a material of the GaInPAs system functionsas an etching stopper layer to the etching process of a material of theAlGaAs system. Thus, by using a wet etching process by a sulfuricetchant for the formation of the mesa structure, it is possible tocontrol the height of the mesa structure rigorously.

[0497] Because of this, the homogeneity and reproducibility are improvedsubstantially for the surface-emission laser diodes that are formedsimultaneously on a substrate and the production cost is reduced. Thisfeature is particularly advantageous when producing a laser diode arrayin which a number of surface-emission laser diodes are integrated inone-dimensional or two-dimensional array.

[0498] In the embodiment of FIG. 27, it is noted that theGaxIn1-xPyAs1-y (0<x≦1, 0<y≦1) layer 17 that acts also as an etchingstopper layer is provided on the side of upper part reflector 18.Further, a similar GaInP layer 13 is provided on the lower partreflector 12.

[0499] In this embodiment, too, the active region 15 sandwiched betweenthe upper and lower reflectors 12 and 18 and cause recombination uponinjection of carriers is formed of a material free from Al. Further, thelow refractive index layer of the lower and upper reflectors 12 and 18closest to the active layer 15 is formed of the non-opticalrecombination elimination layer 13 or 17 having a compositionrepresented by GaxIn1-xPyAs1-y (0<x≦1, 0<y≦1). Thus, Al is not containedat the interface between the active region 15 and the reflector 12 or18, and the problem of non-optical recombination caused by the crystaldefects, which in turn are caused by Al, is effectively eliminated.

[0500] While such a construction of using an Al-free material at theinterface between the reflector and the active region is preferablyprovided to both of the upper and lower reflectors 12 and 18, theadvantageous effect of the present invention is obtained also in thecase such a construction is used on only one of the upper and lowerreflectors 18 and 12. Further, it is also possible to use asemiconductor Bragg reflector in only one of the reflectors and form theother reflector by a dielectric reflector.

[0501] As the lower part reflector 12 provided between the GaAssubstrate 11 and the active layer 15 is constructed similarly to thecase of FIG. 1 in the present embodiment, the adversary effects ofcrawling up of the crystal defects, originating from Al, into the activelayer is effectively suppressed. Thereby, the active layer 15 can beformed with high quality.

[0502] It should be noted that such a non-optical recombinationelimination layer 13 or 17 constitutes a part of the semiconductor Braggreflector 12 or 18 in any of the constitution of FIG. 1 or 27, and thus,the thickness thereof is set to ¼ the oscillation wavelength λ asmeasured in the medium (λ/4). It is also possible to provide such anon-optical recombination elimination layer in plural numbers.

[0503] In the example above, the non-optical recombination eliminationlayer 13 or 17 is provided to a part of the semiconductor Braggreflector 12 or 18. On the other hand, it is also possible to providesuch a non-optical recombination elimination layer inside the resonator15. In the case the resonator 15 is formed of the active layer 15consisting of the GaInNAs quantum well layer 15 a and the GaAs barrierlayer 15 b, for example, it is possible to use the GaAs layer as thefirst barrier layer and the non-optical recombination elimination layerof GaInPAs, GaAsP or GainP as the second barrier layer. Thereby, it ispossible to set the thickness of the resonator to one wavelength. Inthis case, because of the large bandgap of the non-optical recombinationelimination layer as compared with the first GaAs barrier layer, theactive region in which carrier injection takes place is substantiallylimited up to the region of the GaAs barrier layer.

[0504] In the case that the process of removing the residual Al sourcematerial or Al reactant, Al compound or Al during the fabricationprocess of the laser diode, it is possible to provide such a processduring the step of forming the non-optical recombination eliminationlayer. Alternatively, the removal of residual Al may be conducted in aprocess of growing a GaAs layer interposed between the process offorming the Al-containing layer and the process of forming thenon-optical recombination elimination layer.

[0505] According to the present embodiment, it becomes possible to forma highly efficient, reliable surface-emission laser diode. Thereby, itis possible to achieve the foregoing advantageous effect of the presentinvention without increasing the number of stacks in the reflector, asthe non-optical recombination elimination layer formed of the Al-freeGaxIn1-xPyAs1-y (0<x≦1, 0<y≦1) layer is used in only the low refractiveindex layer located closet to the active region in the semiconductorBragg reflector 12 or 18.

[0506] Further, it should be noted that filling of polyimide is easilymade, and thus, it is possible to provide interconnection wiring formingthe p-side electrode, without risking the disconnection of the wiring atthe stepped part. Thereby, the reliability of the device is improvedfurther.

[0507] It was confirmed that the surface-emission laser diode thusproduced oscillates at the laser oscillation wavelength of about 1.3 μm.

[0508] In the present embodiment, it became possible to construct asurface-emission laser diode of long wavelength band on the GaAssubstrate 11 by using a GaInNAs layer containing Ga, In, N and As forthe major element. By forming a current confinement structure byapplying a selective oxidation process to the AlAs layer 181 to form theoxide current confinement layer 182, the threshold current of laseroscillation is reduced effectively. In such a current confinementstructure, formed of the AlAs layer 18, in which the selective oxidationlayer 182 is formed, it is possible to provide the current confinementstructure close to the active layer, and lateral diffusion of theinjected electric current is suppressed effectively. As a result of useof such a current confinement structure, it becomes possible to confinethe careers effectively into a minute region not exposed to theatmosphere. The foregoing current confinement structure has anotheradvantageous feature in that the Al oxide constituting the layer 182 hasa low refractive index and the efficiency of carrier confinement isincreased further. Thereby, the efficiency of laser oscillation isimproved furthermore and the threshold electric current is reduced.Further, the current confinement structure has an advantageous featurein that formation thereof is made easily and the production cost isreduced.

[0509] From the explanation noted above, the laser diode of FIG. 27oscillates efficiently at the wavelength of 1.3 μm similarly to the caseof FIG. 1. The laser diode of FIG. 27 also has advantageous feature ofsmall power consumption and low production cost.

[0510] The surface-emission laser diode of FIG. 27 can be formed also byan MOCVD process, similarly to the case of FIG. 1, However, it is alsopossible to use an MBE process or other growth process. Further, it ispossible to use nitrogen or a nitrogen compound such as NH3 in anactivated state.

[0511] Furthermore, it is possible to replace the triple quantum wellstructure (TQW) in the active layer 15 with other structure includingdifferent number of quantum wells such as SQW structure, DQW structureor MQW structure. Further, it is possible to use a laser diode ofdifferent structure.

[0512] By adjusting the composition of the GaInNAs active layer 15 a inthe surface-emission laser diode of FIG. 1, it becomes possible torealize a surface-emission laser diode of the 1.55 μm band and furtherthe 1.7 μm band. In the present invention, the GaInNAs active layer maycontain other III-V elements such as Tl, Sb, P, and the like. Further,it is also possible to construct a surface-emission laser diode of 1.3μm band on a GaAs substrate by using GaAsSb for the active layer.

[0513] In the description heretofore, it was assumed that the activelayer contains Ga, In and Ss as the major elements (GaInAs active layer)or Ga, In, N and As as the active layer (GaInNAs active layer). Further,the active layer may be formed of any of GaNAs, GaPN, GaNPAs, GaInNP,GaNAsSb, and GaInNAsSb. The present invention is particularly effectivealso in these cases in which the active layer contains N.

[0514]FIG. 28 shows the room temperature photoluminescence spectrumobtained for the active layer that includes a GaInNAs/GaAs doublequantum well structure formed by a GaInNAs quantum well layer and a GaAsbarrier produced by an MOCVD process. Further, FIG. 29 shows thestructure of the sample used for the measurement.

[0515]FIG. 29 is referred to.

[0516] It can be seen that there are formed a lower part cladding layer202, an intermediate layer 203A, an active layer 204 containing thereinnitrogen, another intermediate layer 203B, and an and an upper partcladding layer 205 consecutively on the GaAs substrate 201.

[0517] Next, FIG. 28 is referred to.

[0518] The curve A shows the result with regard to the sample that usesan AlGaAs layer as the cladding layer 202 and the double quantum wellstructure is formed while interposing a GaAs layer similar to theintermediate layer 203A or 203B between the quantum wells. The curve B,on the other hand, shows the measurement result with regard to thesample that uses a GaInP layer as the cladding layer 202 and the doublequantum well structure is formed continuously while interposing a GaAslayer similar to the intermediate layer 203A or 203B.

[0519] As can be seen in FIG. 28, the photoluminescence intensity of thesample A has fallen off by one-half or more with respect to thephotoluminescence intensity of the sample B. This means that therearises a problem of decrease of photoemission efficiency of the activelayer when the active layer containing nitrogen therein such as GaInNAsis grown continuously on a semiconductor layer that contains Al asconstituent element such as AlGaAs while using a single MOCVD apparatus.Because of this, the threshold current density of the GaInNAs laserdiode formed on an AlGaAs cladding layer is increased by twice or moreas compared with the case in which the same laser diode is formed on anGaInP cladding layer.

[0520] The inventor of this invention conducted through investigation ofthis problem and obtained a knowledge explained below.

[0521]FIG. 30 shows the depth profile of nitrogen and oxygen in thedevice for the case the cladding layers 202 and 205 are formed by anAlGaAs layer, the intermediate layers 203A and 203B are formed of GaAs,and the active layer is formed of the GaInNAs/GaAs double quantum wellstructure and the device is formed by using a single MOCVD apparatus. Itshould be noted that the measurement of FIG. 30 was conducted under themeasurement condition summarized in Table 1 by secondary ion massspectrometry. TABLE 3 first ion specie Cs+ first acceleration voltage3.0 kV sputter rate 0.5 nm/s measurement area 160 × 256 μm² degree ofvacuum <3E−7Pa polarity of measured ion −

[0522]FIG. 30 is referred to.

[0523] It can be seen that there appear two nitrogen peaks in the activelayer 204 in correspondence to the GaInNAs/GaAs double quantum wellstructure. Further, it is also noted that an oxygen peak is detected inthe active layer 204. On the other hand, the Oxygen concentration in theintermediate layers 203A and 203, which are free from N and Al, is lowerin terms of one order as compared with the oxygen concentration in theactive layer 204.

[0524] Further, the depth profile of oxygen measured about the samplehaving a constitution in which the active layer 204 including thereinthe GaInNAs/GaAs double quantum well structure is formed on the claddinglayer of GaInP together with the GaAs intermediate layers 203A and 203B,has revealed that the oxygen concentration in the active layer 204 is inthe background level.

[0525] Thus, it was confirmed experimentally that the problem of oxygenincorporation into the active layer 204 occurs singularly in the case inwhich the laser diode has the nitrogen-containing active layer 204 inthe state that the active layer 204 is separated from the substrate bythe semiconductor layer 202 containing Al and the growth of the layersconstituting the device is conducted by a single MOCVD apparatus whileusing a nitrogen compound source material and an organic Al sourcematerial. The oxygen atoms thus incorporated into the active layer forma non-optical recombination state therein and cause a decrease ofluminous efficacy of the active layer.

[0526] Thus, from the result of FIG. 30, it is concluded that oxygenincorporated into the active layer is the cause the problem of deceaseof optical emission efficiency in the device in which the Al-containinglayer is provided between the substrate and the N-containing activelayer. It is conceivable that the origin of this oxygen contaminationwould be the oxygen-containing material that has captured residualoxygen in the deposition apparatus or the impurity contained in thenitrogen compound source material.

[0527] Next, the mechanism of oxygen incorporation into the active layer204 will be examined.

[0528]FIG. 31 shows the depth profile of Al obtained for the same sampleof FIG. 30. The measurement was conducted by the secondary ion massspectroscopy similarly to the case of FIG. 30 under the measurementcondition shown in Table 2. TABLE 4 first ion specie O2+ firstacceleration voltage 5.5 kv sputter rate 0.3 nm/s measurement area 60μmφ degree of vacuum <3E−7OPa polarity of measured ion +

[0529]FIG. 31 is referred to.

[0530] It can be seen that Al is detected in the active layer 204, whichwas formed without introducing an Al source material. Further, it isnoted that the Al concentration in the GaAs intermediate layer 203A or203B formed adjacent to the Al-containing cladding layer 202 or 205 islower than the Al concentration in the active layer 204 by the order ofone. This indicates that Al in the active layer 204 has caused diffusionfrom the Al-containing cladding layer 202 or 205 and has substituted Gain the active layer 204.

[0531] On the other hand, Al was not detected in the active layer in thecase that the N-containing active layer was grown on a semiconductorlayer free from Al such as GaInP.

[0532] From this, it is concluded that Al detected in the active layer204 originates from residual Al such as residual Al source material orresidual Al reactant or residual Al compound or residual Al remaining inthe growth chamber or gas supply line and has been incorporated into theactive layer as a result of coupling with the nitrogen source compoundor the impurity such as water contained in the nitrogen source compound.Further, it is concluded that the Al incorporation into the N-containingactive layer is inevitable when a semiconductor light emitting devicehaving an Al-containing layer between the substrate and the N-containingactive layer is grown continuously by a single epitaxial apparatus.

[0533] By comparing FIG. 31 with FIG. 30, it is noted further that thetwo oxygen peaks in FIG. 30 corresponding to the double quantum well donot coincide with the two nitrogen peaks but coincide with the peakprofile of Al shown in FIG. 31. This results indicates that oxygen inthe GaInNAs quantum well layer has been incorporated into the activelayer not with the nitrogen source but with Al in the form coupled withresidual Al that has been incorporated into the quantum well layer.

[0534] More specifically, the residual Al source material or residual Alreactant or residual Al compound or residual Al in the MOCVD chamber isthought to cause a coupling with water contained in the nitrogen sourcecompound or oxygen-containing material remaining in the gas line orreaction chamber and is incorporated into the active layer in the formcoupled with oxygen or water.

[0535] Thus, it was clarified by the inventor of the present inventionthat, in the surface-emission laser diode of a conventional GaAs system,there occurs oxygen incorporation into the active layer and oxygen thusincorporated has caused the problem of decrease of efficiency of opticalemission.

[0536] The foregoing discovery provides a clue to the improvement ofefficacy of the surface-emission laser diode of the GaAs system in thatit is necessary to remove the impurity at least from the part of thedeposition apparatus used for the production of the laser diode and mayhave a chance to make a contact with the nitrogen compound sourcematerial in the deposition chamber or the impurity contained in thenitrogen compound source material.

[0537] Thus, by providing a step of removing the residual Al after thegrowth of the Al-containing layer but before the growth of theN-containing active layer, it becomes possible to reduce theconcentration of Al and oxygen impurity incorporated into the activelayer. By doing so, the concentration of residual Al source material orAl reactant or Al compound or Al concentration is already reduced whenthe nitrogen compound source material is supplied for the growth of theN-containing active layer, and the reaction between the residual Al andthe nitrogen source compound or the impurity contained in the nitrogensource compound is successfully suppressed.

[0538] It is further advantageous to remove the residual Al before theend of the growth of the non-optical recombination layer. By doing so,it is possible to suppress the non-optical recombination of carriers inthe active layer at the time carriers are injected into the active layerby way of current injection.

[0539] For example, it became possible to carry out a continuousoscillation of the surface-emission laser diode of the GaAs system atroom temperature environment by reducing the Al concentration in theN-containing active layer to 1×10¹⁹ cm⁻³ or less. Further, by reducingthe Al concentration in the N-containing active layer to 2×10¹⁸ cm⁻³ orless, optical emission characteristics substantially equal to the casewhen the active layer is grown on an Al-free semiconductor layer isobtained.

[0540] The removal of the residual Al source material or residual Alreactant or residual Al compound or residual Al from the part of thedeposition chamber that may have a chance to make a contact with thenitrogen compound source material or the impurity contained in thenitrogen compound source material, can be advantageously conducted by apurging process that uses a carrier gas as the purge gas. Here, the timeof the purging process is defined as the time after the supply of the Alsource material into the deposition chamber is stopped in correspondenceto completion of the growth of the Al-containing layer but before thestart of supply of the nitrogen compound source material to thedeposition chamber for the start of growth of the N-containingsemiconductor layer. It is also possible to interrupt the growth of theintermediate layer, which is free from any of Al and nitrogen, andconduct a purging process by using the carrier gas as a purging gas. Inthis case, the interruption of the growth process of the intermediatelayer may be achieved after the start of growth of the Al-containinglayer until the midway of the growth of the non-optical recombinationlayer.

[0541]FIG. 32 shows an example of the semiconductor light-emittingdevice formed by providing such a carrier purging process. In FIG. 32,those parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

[0542] Thus, it can be seen that the Al-containing first semiconductorlayer 202, the first lower part intermediate layer 601, the second lowerpart intermediate layer 602, the N-containing active layer 204, theupper part intermediate layer 203, and the second semiconductor layer205 are stacked consecutively the on substrate 201 in FIG. 32.

[0543] In forming the structure of FIG. 32, the crystal growth iscarried out by an epitaxial growth apparatus by using an organic-metalAl source and an organic nitrogen source material. In the case, thegrowth interruption process is provided before the start of growth ofthe second lower part intermediate layer 602 but after the growth of thefirst lower part intermediate layer 601. During the growth interruptionprocess, the part of the growth chamber that may make a contact with thenitrogen compound source material or the impurity in the nitrogencompound source material is purged by a hydrogen gas used as the carriergas, such that the residual Al source material or residual Al reactantor residual Al compound or residual Al is removed.

[0544]FIG. 33 shows the result of measurement of the depth profile of Alconcentration on a semiconductor light-emitting device in which therehas been provided a growth interruption between the first lower partintermediate layer 601 and the second lower part intermediate layer 602and a purging process was conducted for 60 seconds.

[0545]FIG. 33 is referred to. It can be seen that the Al concentrationin the active layer 204 is reduced to 3×10¹⁷ cm⁻³ or less as a result ofsuch a growth interruption process and purging process. This value of Alconcentration is the same degree as the Al concentration in theintermediate layers 601 and 602.

[0546]FIG. 34 shows the depth profile of nitrogen and oxygen for thedevice of FIG. 32.

[0547]FIG. 34 is referred to. It can be seen that the oxygenconcentration in the active layer 204 is reduced to 1×10¹⁷ cm⁻³, whichis a background level. The oxygen peak appearing in the lower partintermediate layer 601 or 602 in FIG. 34 is interpreted as showingoxygen segregation to the growth interruption interface as a result ofinterruption of growth. Thus, it is preferable to conduct the growthinterruption period after the termination of the growth of thesemiconductor layer containing Al but before the growth termination ofthe non-optical recombination elimination layer, in the case there isprovided a growth interruption and purging process. In the non-opticalrecombination elimination layer, it is possible to increase the bandgapenergy as compared with the quantum well active layer or barrier layer,and the adversary effect of the non-optical recombination caused by theoxygen segregated to the growth interruption interface is effectivelysuppressed when career injection is made into the active layer. Theconstitution that uses the non-optical recombination elimination layerprovides a particularly advantageous effect in the case of using anactive layer containing nitrogen.

[0548] In the semiconductor light-emitting device of FIG. 32, theimpurity concentration level of Al and oxygen in the nitrogen-containingactive layer 204 is reduced successfully by interrupting the growthprocess between the first lower part intermediate layer 601 and thesecond lower part intermediate layer 602 and by conducting a purgingprocess for 60 minutes. In this way, the optical efficacy of the activelayer 204 was effectively improved.

[0549] Further, it is also possible to carry out the Al removal processwhile heating the susceptor during the purging process conducted with acarrier gas. By doing so, the Al source material or reaction productadsorbed on the susceptor or the region near the susceptor iseffectively decoupled and removed. In the case of conducting such aheating of the substrate during the purging process, it is necessary tosupply the group V source gas such as AsH3 or PH3 so as to avoid thermaldecomposition of the uppermost semiconductor layer.

[0550] It is also possible to transport the substrate to a chamberdifferent from the growth chamber when conducting the purging processwith a carrier gas in an MOCVD apparatus. In the case that the substrateis transported to another chamber from the growth chamber, it is notnecessary to keep supplying the group V source gas such as AsH₃ or PH₃to the growth chamber. Thereby, the efficiency of thermal decompositionof the Al reactant deposited on the susceptor or the region surroundingthe susceptor is substantially facilitated and the efficiency of removalof Al is improved.

[0551] Further, it is also possible to carry out the purging processwhile continuing the growth of the intermediate layer. In theconstitution of FIG. 27, for example, the non-optical recombinationelimination layer 13 is provided between the N-containing active layer15 and the reflector 12 formed of the AlGaAs material containing Al.Thereby, the distance between the N-containing active layer 15 and theAl-containing layer is increased, and it is possible to increase theduration of the purging process in the case the purging process isconducted simultaneously to the growth. In such a case, it isadvantageous to decrease the growth rate and increase the purging time.

[0552] Further, it is also possible to carry out the growth of theAl-containing reflector 12 of AlGaAs material and the N-containingactive layer 15 in respective, separate apparatuses. In this case, too,the impurity concentration level of Al or oxygen in the active layer 15can be reduced, by setting the regrowth interface below the non-opticalrecombination elimination layer 13.

[0553] In the case that the surface emission laser diode is producedwith the crystal growth process that does not use an organic-metal Alsource or nitrogen compound source material such as an MBE process,there is no report about the degradation of the optical efficiency inthe semiconductor light-emitting device in which N-containing activelayer is provided on the Al-containing semiconductor layer. In case ofan MOCVD process, on the other hand, degradation of optical efficacy ofthe GainNAs active layer formed on the Al-containing semiconductor layeris reported.

[0554] In Electron. Lett., 2000, 36 (21), pp. 1776-1777, for example,there is a report that the photoluminescence intensity deterioratesremarkably when a GaInNAs quantum well layer is grown continuously on anAlGaAs cladding layer in the same MOCVD growth chamber, even when anintermediate layer of GaAs is provided on the AlGaAs cladding layer. Inthe foregoing report, the GaInNAs active layer is grown in a MOCVDchamber different from one used for growing the AlGaAs cladding layer soas to improve the photoluminescence intensity.

[0555] Thus, it is believed that the aforementioned problem arises moreor less when growing a crystal layer by an organic-metal Al source and anitrogen compound source, as in the case of an MOCVD process.

[0556] In the MBE process, the crystal growth is carried out under aultra low-pressure environment (high vacuum state). On the other hand,an MOCVD process is conducted under a process pressure of several tenTorr to the atmospheric pressure. Thus, the mean free path of thegaseous molecules is overwhelmingly short in an MOCVD process ascompared with the MBE process. Because of this, it is conceivable thatthe source gas molecules or the carrier gas molecules make a contactwith various parts of the gas line, reaction chamber, and the like.

[0557] Thus, in the case of the growth process that uses a relativelyhigh pressure for the reaction chamber or gas line as in the case of theMOCVD process, it becomes possible to eliminate the incorporation ofoxygen into the N-containing active layer, by providing a removal stepfor removing the residual Al source material or Al reactant or Alcompound or Al from the location of the reaction chamber, in which thereaction chamber may make a contact with the nitrogen compound sourcematerial or the impurity contained in the nitrogen compound sourcematerial. Such a removal step should be conducted after the growth ofthe Al-containing semiconductor layer but before the start of growth ofthe N-containing active layer. More preferably, the removal processshould be carried out after the growth of the Al-containingsemiconductor layer but before the end of the growth of the non-opticalrecombination elimination layer.

[0558] For instance, it is possible to evacuate the gas line or thegrowth chamber with a vacuum evacuation process after the growth of theAl-containing semiconductor layer but before the growth of theN-containing active layer. In this case, it is preferable to carry outthe evacuation process in the state that the substrate is heated.

[0559] Also, it is possible to remove the residual Al after the growthof the Al-containing semiconductor layer but before the growth of theN-containing active layer by supplying an etching gas. For example, anorganic compound gas may be used as the etching gas that reacts with theAl-containing residue.

[0560] For example, it is possible to cause a reaction in theAl-containing residue by supplying a DMHy gas, which is one of theorganic compound gases, at the time of the growth of the active layerfrom a DMHy cylinder. Therefore it is possible to remove theseAl-containing residue by causing a reaction between the Al-containingresidue remaining on the reaction chamber surface, heating zone, or jigsused for supporting the substrate, and the organic compound gas bysupplying the organic compound gas from a gas cylinder, after the growthof the Al-containing semiconductor layer. By a method, too, it ispossible to suppress the incorporation of oxygen into the active layer.Particularly, by using the same gas as the gas used for the nitrogensource it is possible to avoid the problem of providing a special gasline. This process can be carried out while interrupting the growth orgrowing a dummy layer containing nitrogen such as GaNAs, GaInNAs, GaInNPseparately from the growth of the active layer. By doing conducting theAl removal process simultaneously to the crystal growth process, it ispossible to reduce the time loss as compared with the case ofinterrupting the growth process, and the throughput of the semiconductordevice production is improved.

[0561] By using GaInAs in the active layer of laser diode, it becamepossible to realize a laser diode oscillating at the wavelength of 1.2μm by growing a highly strained GaInAs quantum well active layer byusing a low temperature growth process of 600° C. This wavelength valueis substantially longer than the wavelength of 1.1 μm, which has beenconceived as the upper limit of laser oscillation wavelength.Conventionally, there was no material suitable for the laser diode of1.1-1.7 μm. On the other hand, it became possible to realize a highlyefficient surface-emission laser diode operable in the wavelength bandof 1.1-1.7 μm, by using a highly strained active layer of GaInAs,GaInNAs or GaAsSb together with the use of the non-optical recombinationelimination layer. With this, the possibility of applications to theoptical-fiber telecommunication systems was opened.

[0562] [Third Embodiment]

[0563]FIG. 35 shows a laser diode chip 32 including the long-wavelengthsurface-emission laser diode of FIG. 1 or FIG. 27 explained previously.The example that formed on an n-type GaAs wafer 31 of a surfaceorientation (100) is shown.

[0564]FIG. 35 is referred to.

[0565] It can be seen that a number of laser diode chips 32 are formedon the GaAs wafer, and each of the laser diode chips 32 carries thereonn laser diodes each having a construction explained before. The number nand arrangement thereof are decided according to the use of laser diodechip 32.

[0566]FIG. 36 shows the example of the optical transmission/receptionsystem using the long-wavelength surface-emission laser diode having thelaser oscillation wavelength of 1.1-1.7 μm band. Those parts explainedpreviously are designated by the same numerals and explanation isomitted.

[0567]FIG. 36 is referred to.

[0568] The surface-emission laser diode chip 32 is provided at the Apoint as an optical source such that the emission part 32A of the laserdiode chip 32 makes an optical coupling with the optical fiber 33. Theoptical signal emitted from the emission part 32A of the laser diodechip 32 is injected into the optical fiber 33. The optical signal istransferred in the direction shown with the thick arrow in FIG. 36. Theend point of the optical fiber 33 is located at the B point. Thephotodiode or other suitable photodetection device 34, which constitutesthe photodetection unit is provided in the B point, and the opticaldetection part 34A thereof is coupled optically to and the optical fiber33. Thus, the optical transmission/reception system is composed.

[0569] In this example, the location B for the photodetection unit andthe location A for the optical source are tied with a straight line bythe optical fiber 33.

[0570]FIG. 37 shows the construction of FIG. 36 schematically.

[0571]FIG. 37 is referred to.

[0572] The black circles A, B in the figure show the location of theoptical source 32 and the photodetection unit 34 respectively. A blackthick line shows the optical fiber 33.

[0573] Conventionally, optical source 32 and photodetection unit 34 areconnected optically with optical fiber 33 that provides the transmissionpath and constitute an optical transmission/reception system. Thetransmission path is formed extending from several tens of meters toseveral tens of kilometers in the optical transmission/reception systemwith the long-wavelength surface-emission laser diode of laseroscillation wavelength 1.1-1.7 μm band of this invention. Thereby, theremay be obstacles between the points A and B.

[0574] For example as for FIG. 38, obstacles 35A and 35B exist betweenthe points A and B. Thus, the point B for the location of thephotodetection unit and the point A for the location of the opticalsource cannot be connected with the transmission path 33 of a straightline. Thus, FIG. 38 shows the example of bending the transmission pathby a right angle.

[0575] When the optical fiber 33 is bent at the point where thetransmission path is bent by a right angle with the same angle as in thecase of FIG. 38, the optical fiber 33 is damaged and stops functioningas a transmission path. It is possible to provide a reflection part inthe bending part of the transmission path so as to deflect the opticalbeam. The use of the reflection part in such a bending part of thetransmission path increases the cost.

[0576] Thus, the present invention proposes an approach to bend theoptical transmission path without providing a reflection part andwithout spoiling the functioning of the optical fiber 33 as atransmission path.

[0577]FIG. 39 shows a third embodiment of this invention. Those partsexplained previously are designated by the same reference numerals andexplanation is omitted.

[0578]FIG. 39 is referred to. Obstacle 35C exists between the A pointand B point. The location B and the location A for the photodetectionunit and the optical source 32, respectively, can not be connected by atransmission path of straight line. However, as shown in thisembodiment, the location B for the photodetection unit 34 and thelocation A for the optical source 32 are connected by bending opticalfiber 33, which composes a transmission path without causing a localangle. Thus, the optical fiber 33 composing the transmission path is notdamaged by the bending, and it is possible to avoid the obstacle 35C.According to the present invention it is possible to construct a fineoptical transmission/reception system even when obstacles must beavoided.

[0579]FIG. 40 is another example according to this invention. In thiscase obstacles 35D, 35E exist between the points A and B. Location B andlocation A of photodetection unit 34 and optical source 32,respectively, can not be connected with a straight line transmissionpath.

[0580] However, by bending the optical fiber 33 of the transmissionpath, without causing a local angle, as shown in this example of thisinvention it is possible to connect the location B for thephotodetection unit and the location A for the optical source. As aresult, the transmission path is bent continuously in this example ofthe present invention. It is not formed step-like and a local angle isnot required.

[0581] Furthermore the above explanation pertains to the opticaltransmission/reception system using the long-wavelength surface-emissionlaser diode with laser oscillation wavelength of 1.1-1.7 μm band tosuitably provide middle to long distance telecommunication systems.According to this invention it is possible to deploy this system insideof a building, for example, even in the case of constructing an opticaltransmission/reception system with a length of several centimeters toseveral meters. Even in this case, the transmission direction of theoptical beam can be changed in all directions without providingreflection part, by bending the optical fiber along the transmissionpath, without causing a local angle therein. According to evaluationsconducted by the inventor of the present invention, there was not anycase wherein the function of a transmission path failed due to theoptical fiber 33 being bent, even in the case that the transmission pathwas bent continuously as indicated in FIG. 40. There was no damage tothe transmission paths such as those shown in FIG. 39 and FIG. 40, aslong as the diameter of the curvature of the transmission path does notexceed 20 cm. To put it differently, the transmission path can bearranged to avoid obstacles such as shown in FIG. 40, even if reflectionpart or the like, that changes the direction is not provided, if thediameter of the transmission path curvature does not exceed 20 cm. Thus,the present invention provides a fine optical transmission/receptionsystem that functions with high reliability at a low cost.

[0582] [Fourth Embodiment]

[0583] Next the fourth embodiment of this invention is explained.

[0584]FIG. 41 shows another example of an optical transmission/receptionsystem using the long-wavelength surface-emission laser diode with laseroscillation wavelength of 1.1-1.7 μm band. Those parts explainedpreviously are designated by the same reference numerals and explanationis omitted.

[0585]FIG. 41 is referred to. In this example the laser diode emissionpart 32 A is coupled optically to an optical fiber F1. The laser beamthat is emitted from the laser diode emission part 32 A is transferredin the direction of the thick arrow in FIG. 41. The laser beam that wastransmitted through the optical fiber F1 is then bent 90 degrees by areflection part R. The laser beam then enters optical fiber F2. Opticalfiber F2 at the end point couples optically to the light detector part34A of the photodetection device 34 such as a photodiode. Thus theoptical transmission/reception system is composed.

[0586] It should be noted that the foregoing description is merely forthe exemplary purpose of the optical transmission/reception system ofthis invention. Thus, only one laser diode emission part 32A is shown inthe present example. However, it also is possible to construct, by usingthe advantageous feature of the long-wavelength surface-emission laserdiode of the present invention, to construct a large capacity opticaltransmission/reception system of the multiple laser array type, in whicha number of laser diodes 32A are formed on one laser diode chip 32A andthe optical transmission/reception system is constructed by using anumber of optical fibers F1 and photodetection devices 34 coupledoptically to the laser diodes 32A.

[0587]FIG. 42 shows another arrangement of the opticaltransmission/reception system of FIG. 41. The opticaltransmission/reception system of FIG. 41 may be a yard, whereas in FIG.42 it is arranged inside a wall of a building. Those parts explainedpreviously are designated by the same reference numeral and explanationis omitted.

[0588]FIG. 42 is referred to.

[0589] A room 42 is defined with a wall 41 by which the building isformed. Within a space 41 A inside the wall 41, a long-wavelengthsurface-emission laser diode chip 32 having laser oscillation wavelengthof the 1.1-1.7 μm band is provided at the A point as an optical source.Further, The photodiode that constitutes the photodetection unit 34 isprovided in the B point Also the reflection part R is provided in thespace 41A so as to deflect the transmission path between the A point andthe B point. FIG. 42 shows the plan view of the room and the opticaltransmission/reception system merely for the purpose of explanation,Because of this, the room or the wall and the opticaltransmission/reception system are not drawn to scale.

[0590] Furthermore, though not indicated by FIG. 42, in each opticaltransmission/reception unit 32 and 34 at the points A and B, there maybeother connect instruments or connectors. These may be in the internalspace 41 A of the wall 41, or may be in the room 42.

[0591]FIG. 43 shows the plan view of an optical transmission/receptionsystem in the room 42. Those parts explained previously are designatedby the same reference numerals and explanation is omitted.

[0592]FIG. 43 is referred to. The laser diode 32 and the photodiode 34are tied with one straight line by the optical fiber F12 crossing theinside of room 42. Therefore the optical transmission/reception systemis arranged on the floor, under the floor, or in the ceiling of theroom.

[0593]FIG. 44 shows another example of the opticaltransmission/reception system in the room 42. Those parts explainedpreviously are designated by the same reference numerals and explanationis omitted.

[0594]FIG. 44 is referred to.

[0595] In this case the laser diode 32 and the photodiode 34 are tied bythe optical fiber F12 by curving the optical fiber F12 while utilizingthe flexibility of the optical fiber.

[0596] However, the optical fiber F12 has to be arranged so as to crossthe room 42, but it can change the direction of the transmission pathonly with big curvature even if it uses the flexibility of opticalfiber. However, the optical fiber has to be bent with a large radius ofcurvature, and the optical transmission/reception system has to beconstructed to or under the floor or in the ceiling of room 42 in thestate that the optical fiber 12 crosses the room 42. Thus, it isnecessary to dispose the optical fiber under the floor or on the flooror on the ceiling.

[0597] In the arrangement shown in FIGS. 26 and 27 the optical fiber hasto be arranged so as to cross the room 42. Therefore, the optical fiberon the floor, under the floor or in the ceiling of the room is arrangedin a very complicated and disorderly manner. It gets more complicatedwhen a plural optical transmission/reception system is arranged. Thenthere are the problems of intertwisting, and maintenance afterinstallation become difficult. Especially, such an arrangement is verydangerous in the case the optical fiber F12 is arranged on the floorwhere a pedestrian can hook the foot.

[0598] The optical fiber transmission path of this embodiment of thepresent invention is well-suited to the wall 41 or pillar or other partsof the building, because the optical fiber transmission path can be bent90 degrees by using the reflection part R as shown in FIG. 42 and isotherwise arranged beautifully in appearance, even if it is arranged ina place where it is not inside of wall 41 and can be seen by the eye. Itgets complicated if a plural optical transmission/reception system isarranged, but there is not a problem of the optical fibersintertwisting. The maintenance after installation can be done easily.

[0599] The reflection part R for the deflection of the transmission pathis provided, in constructing such a peripheral (out of sight) opticaltransmission/reception system in this embodiment. By using thereflection part R, the transmission path is able to be arrangedeffectively. Instead of exposing the transmission path unnecessarily,the reflection part material R puts the transmission path inside theceiling, under the floor or in the wall of a building. The transmissionpath does not occupy area needlessly. The building design comes to beproduced effectively. The degree of freedom for an aesthetic designincreases.

[0600] Furthermore the reflection part R is not necessarily restrictedto 90 degrees, in the case that the direction of the transmission pathis bent by reflection part material R. However, in the normal case, abuilding is executed/designed, unless there is a demand for a specialdesign, for pillars, walls, floors, and ceilings to provide many caseswhere one straight line crosses another straight line at 90 degrees. Insuch normal cases it is desirable for reasons of esthetic beauty and ofefficiency, when the optical fiber transmission path changes 90 degrees,for it to be arranged along a pillar, wall, floor, ceiling and theothers.

[0601] Also, exposing the optical fiber transmission path to the eyeunnecessarily be avoided in constructing such an opticaltransmission/reception system, by using the reflection part material Rfor changes in direction of the transmission path so that thetransmission path does not occupy area needlessly inside the ceiling,under the floor or in the wall of a building. Thereby the buildingdesign comes to be produced effectively and the degree of freedom foraesthetic design increases.

[0602]FIG. 45 shows an example of another optical transmission/receptionsystem of this embodiment. The optical signal emitting from the emissionpart 32 A of the laser diode 32, in this example, is transferred inspace. It goes straight along the arrow of FIG. 45. It is received withthe light detector part 34 A comprised of a photodiode and otherphotodetection devices 34.

[0603] As explained according to the embodiments of the presentinvention previously, it is preferable to use a surface-emission laserdiode equipped with the improved semiconductor Bragg reflectors 12 and18 further with the non-optical recombination elimination layer andhaving the laser oscillation wavelength band of 1.1-1.7 μm for the laserdiode, in view point of reliability, low power driving, and lowproduction cost. The foregoing wavelength was not possible before in asurface emission laser diode.

[0604] The following description will be made for the construction inwhich there is only one laser diode. However, it should be noted that,in the case of the long-wavelength surface-emission laser diode of the1.1-1.7 μm band, a number of laser diodes are formed on a single chipwith low cost, and it is easy to construct a multiple laser arraysystem. Thereby, a large capacity telecommunication system us realized.

[0605]FIG. 46 is another example of the optical transmission/receptionsystem of this embodiment. Those parts explained previously aredesignated by the same reference numerals and explanation is omitted.

[0606]FIG. 46 is referred to.

[0607] The optical signal emitted from the emission part 32A of laserdiode 32 is transferred through space. It goes straight in the directionof the arrow in FIG. 46. The progress direction of the optical path isbent by a midway, for example by the reflection part R. The opticalsignal is received with the light detector part 34A comprising thephotodetection devices 34 such as a photodiode, and the like.

[0608] [Fifth Embodiment]

[0609] Conventionally the signal transmission of inside an electronicapparatus has been achieved by using a conductor cable in the form ofelectrical signals. Therefore numerous conductor cables are connectedinside various electronic apparatuses. The arrangement/processing of theconductor cable was complicated, presenting problems in the design stageand with the assembling stage at the factory.

[0610] Thereupon, instead of exchanging of the signals by such conductorcable, in this embodiment of this invention the electrical signals aresubstituted for with optical signals by the opticaltransmission/reception system as shown in FIG. 45 or FIG. 46. Theoptical signal that is substituted is transferred through space. Suchsubstitution omits or decreases conductor cable. The conductor cableinside the instrument is decreased as much as possible. Internal wiringis simplified. By simplifying the internal wiring of the electronicapparatus the complexity of the layout and processing of conductor cableis reduced. Reducing or eliminating the complexity can increase thedegree of freedom of each part/unit and other layout items inside theapparatus.

[0611] Electronic apparatuses for which the opticaltransmission/reception system of this embodiment of this invention isincorporated include, for example, recording apparatuses such as acopying machine or laser printer that uses electrophotography, ink-jetrecording apparatus, or recording apparatus using a silver saltphotography process. In addition, the present invention can be used incomputers, video apparatuses, television sets.

[0612] As for FIG. 47, the figure shows an electrophotographic copyingmachine 541 to which this embodiment is applied. FIG. 48 enlarges FIG.47.

[0613]FIG. 48 shows the internal structure of the electrophotographiccopying machine 541 schematically. Those parts explained previously aredesignated by the same reference numerals and explanation is omitted.

[0614]FIGS. 47 and 48 are referred to.

[0615] The electrophotography copying machine 541 has a sheet feedcassette 542 and a sheet recovery tray 543. Furthermore wiring board 550and image formation mechanism 549 including sheet feed mechanism 548 andphotosensitive drum 546 are connected by electrical circuit and includedin the main body case that is closed with cover 547 a and fitted to thecase 547.

[0616] And also the photodiode 34 and the laser diode 32 are located inthe case 547 respectively at the point A and the point B, all previouslyexplained. The laser diode 32 and photodiode 34 are connected optically,by the construction explained with FIG. 41 previously and use opticalfiber F1, the mirror R and optical fiber F2.

[0617]FIG. 49 shows an ink-jet recording apparatus 551, as anotherexample to which the present invention is suitably applied. FIG. 50 isthe figure that enlarges FIG. 49 and schematically shows the internalstructure.

[0618]FIGS. 49 and 50 are referred to.

[0619] The ink-jet recording apparatus 551 has a case 554 comprising alower part case 554 b and an upper part case 554 a. A sheet feedmechanism 555 and an ink-jet recording head 556 are provided on theupper part case 554 a.

[0620]FIG. 50 is referred to. The ink-jet recording head 556 is providedso as to move right and left on a carriage 556A. Image formation isperformed on the recording sheet that is pressed on platen roller 558 byrecording part 557.

[0621] Wiring board 559 provides an electronic circuit in the lower partcase 554 b. Optical fiber F12 similar to the embodiment explained aboveis provided between the A point and B point in the wiring board 559.Optical fiber 12 transfers the optical signal to the photodetectiondevice 34 at the B point from the optical source of the laser diode chip32 provided at the A point.

[0622] According to this embodiment of the present invention, conductorcable in the electronic apparatuses is eliminated or reduced. Conductorcable is replaced inside the instrument by an opticaltransmission/reception system. The examples of an electrophotographycopying machine and ink-jet recording apparatus were shown in FIGS.47-50, as the electronic apparatuses to which this embodiment isapplied. This invention is not restricted to these apparatuses.

[0623] [Sixth Embodiment]

[0624] In the recording apparatuses such as an ink-jet recordingapparatuses or recording apparatus of silver salt photography or anelectrophotographic recording apparatus, there arises a problem in thattoners or ink or liquid developer form a mist. Further, dust particlessuch as paper dust may float inside the apparatus. Thus, the inside ofsuch apparatuses is not an ideal environment for an opticaltransmission/reception system of this invention.

[0625] In consideration this problem, the sixth embodiment of thisinvention shown in FIG. 51 provides a cover member 32B covering thelaser diode chip 32 or a cover member 34B covering the photodiode 34.For example, these cover members 32B and 34B are formed by a transparentglass-like member. In FIG. 51, those parts explained previously aredesignated by the same reference numbers, and explanation thereof willbe omitted.

[0626] It is possible to use a high-precision plastic member, from whichstrain is removed, in addition to a glass, for the cover members 32B and34B. Such a cover member provides physical and chemical protection ofthe laser diode and the photodetection device used in this inventionthat are produced by a sophisticated process.

[0627] Furthermore the toner, ink or paper dust and other foreign mattercannot be allowed to adhere to the laser diode chip of this inventionand spoil the function of the optical transmission/reception system,such as shading of the laser beam. Therefore it is desirable to formthese cover members detachable. By making the cover members detachable,the cover members may be removed right away anytime that foreign matteradheres. Furthermore a cover is not provided to reflection member R inFIG. 51. As occasion demands a cover can be provided also to thereflection member R.

[0628] Considering the contamination originating from toner and paperdusts, it is thought that it would be more effective to provide such acover member 32B or 34B in the apparatuses such as a printer or copyingmachine than a computer or a video apparatus or a television set.

[0629] [Seventh Embodiment]

[0630]FIG. 52 shows an example another optical transmission/receptionsystem that uses the long-wavelength surface-emission laser diode havingthe laser oscillation wavelength of 1.1-1.7 μm band. Those partsexplained previously are designated with the same reference numerals andexplanation is omitted.

[0631]FIG. 52 is referred to. This embodiment concatenates and connectsplural optical fibers along the optical transmission path in theconstruction of FIG. 36. This embodiment enables an optical transmissionpath of long distance. There are three optical fibers in this example.

[0632] The optical telecommunication system was previously studied withthe wavelength of 0.85 μm. However, the transmission loss of the opticalfiber was large at this wavelength and it was not practical. On theother hand, it has been difficult previously to compose thesurface-emission laser diode oscillating stably at a practical longwavelength band in which the transmission loss becomes minimum in theoptical fiber.

[0633] In accordance with this invention, the surface-emission laserdiode that oscillating at the wavelength band of 1.1-1.7 μm is realizedby improving the semiconductor Bragg reflector 12 or 18 and providingthe non-optical recombination elimination layers 13 and 17. Thus apractical long-wavelength band optical telecommunication system is nowpossible.

[0634] In the illustrated example, the laser beam that emitted from thelaser diode emission part 32A of the long-wavelength surface-emissionlaser diode chip 32 is injected into a first optical fiber FG1 fortransmission, and the laser beam exited the first optical fiber FG1 isinjected to a second optical fiber FG2 for further transmission.Furthermore, the laser beam that exited this second optical fiber FG2 isinjected into a third optical fiber FG3 for transmission, wherein a thephotodiode chip 34 having the photodetector part 34A is coupled to thethird optical fiber FG3 for detection. Thus, the optical fibers FG1, FG2and FG3 are disposed along the transmission path of the laser beam.

[0635] Between the laser diode chip 32 and the first optical fiber FG1,there is provided an optical connection module MG1 for connecting thelaser diode and optical fiber. Similarly, optical connection modulesMG2, MG3 and MG4 are provided between the optical fibers or between theoptical fiber and the photodiode chip for optical coupling.

[0636]FIG. 53 shows the bidirectional optical transmission/receptionsystem that has the construction that provides transmission system FGAcorresponding to the system of the above FIG. 52 and transmission systemFGB inverted to the system of the FIG. 52. For the parts in the FIG. 53that are the same as those explained above with the same referencenumerals, explanation is omitted.

[0637]FIG. 53 is referred to.

[0638] The transmission system FGB includes, from the right to the left,a third optical fiber FR3 transmitting the laser beam emitted from thelaser diode part 32A of the surface-emission laser diode chip 32, asecond first optical fiber FR2 transmitting the laser beam emitted fromthe optical fiber FR3, a first optical fiber FR1 transmitting the laserbeam emitted from the optical fiber FR2, and the light detector part 34Aof the photodiode chip 34 is coupled optically to the first the opticalfiber FR1.

[0639] A connect module MR4 is provided between the laser diode chip 32and the third optical fiber FR3 in the transmission system FGB, whereinthe connect module MR4 couples optically the laser diode chip 32 and theoptical fiber FR3. Similarly, there are provided connect modules MR1,MR2 and MR3 are for optical couplings respectively between the opticalfibers FR3 and FR2, between the optical fibers FR2 and FR1, and betweenthe optical fiber FR2 and the photodiode chip 34,

[0640]FIG. 54 shows an example of parallel optical telecommunicationsystem in which n optical transmission/reception systems each having theconstruction of FIG. 52 are arranged in parallel.

[0641]FIG. 54 is referred to.

[0642] The plural laser emission part 32 A is provided on a laser diodechip 32. Further, first, second and third optical fiber groups MFG1,MFG2 and MFG3 are constructed by a number of optical fibers eachcorresponding to one of the plural optical emission parts 32A. Further,there are provided a number of photodetector parts 34A in the photodiodechip 34 in correspondence to the plural laser emission parts 32A.

[0643] In the present invention, in which the surface-emission laserdiode chip is used, it is easy to provide a number of laser emissionparts 32A on the laser diode chip 32. By providing a number of laseremission parts 32A on the laser diode chip, it becomes possible torealize a large capacity telecommunication system easily.

[0644] Furthermore it is possible to construct a large capacitybi-directional optical transmission/reception system that expands theoptical transmission/reception system of FIG. 54, in accordance with theconstruction of the bi-directional optical transmission/reception systemof FIG. 53 and using the optical fibers of plural groups in parallel,although FIG. 57 does not indicate a bi-directional system.

[0645] [Eighth Embodiment]

[0646] Next another embodiment of this invention will be explained.

[0647]FIG. 55 shows the construction of an optical connection module MG1that is used to optically couple the surface-emission laser diode chip32 and the first optical fiber group MFG1 in the left part of FIG. 54.FIG. 55 shows optical connection module MG1 schematically with arectangular dotted line.

[0648] Detailed construction of the optical connection module MG1 willbe explained hereinafter with reference to FIGS. 39-59. Those partsexplained previously are designated by the same reference numerals andexplanation is omitted.

[0649]FIGS. 56 and 57 show the construction of the surface-emissionlaser diode chip 32 and the optical fiber group MFG1 respectively in thestate before they are coupled and the state after they are coupled.

[0650]FIG. 56 is referred to.

[0651] Optical connection module MG1 shown by a rectangular dotted linepart schematically in FIG. 55 is formed of a fiber holder 61 and a fiberholder 62 that holds each of the optical fibers fg1 that constitutes theoptical fiber group FG1. The Fiber holder 62 is inserted into chipholder 61 as shown in FIG. 57 with arrows so as to face each other.Thus, the optical coupling is achieved between each of the laser diodeemission parts 32A and the edge surface of the corresponding opticalfiber fg1. Thereby the desired optical coupling is achieved.

[0652] In the case of this invention, the holders 61 and 62 are providedwith discrimination means such that each of the laser diode emissionpart 32 s A and each of the optical fiber fg1 face with each other inone-to-one relationship, without ambiguity in the lateral or verticaldirections. For this purpose, an arrow mark is provided to each of thelaser diode chip holder 61 and the fiber holder 62 in the example of thedrawing. Thereby, optical connection can be performed very efficiently,because of the ability to discriminate the direction of holders 61 and62 correctly and instantaneously at the time of the connection betweenlaser diode chip 32 and optical fiber group FG1, at the time the opticaltransmission/reception system of the present invention is constructed.

[0653] Furthermore, the discrimination means is not restricted to thearrow mark but it is also possible to use a different color.Furthermore, the discrimination means is not necessarily restricted tothe ones that can be visually recognized. It is possible to have adiscrimination means using the tactile sense by utilizing the unevennessand other features of form. It is possible to discriminate the directionof holders 61, 62 briefly even in the case of construction work indarkness or nighttime, in the case that discrimination means able todiscriminate by the tactile sense is used.

[0654]FIGS. 58 and 59 show a different example of the optical connectionmodule MG1 that consists of the combination of laser diode chip holder61 and fiber holder 62.

[0655]FIGS. 58 and 59 are referred to.

[0656] With the case as shown in FIGS. 39 and 40 this embodimentprovides discrimination means to each of laser diode chip holder 61 andfiber holder 62 similarly. So that it is possible to connect and locateboth holders accurately, in this embodiment a flange surface isadditionally formed so as to allow the edge part of laser diode chipholder 61 engage the fiber holder 62. The flange surfaces 61A and 62Bare shown in FIG. 58.

[0657] Next is explained another feature of this invention with regardto the discrimination means, the location/connection means shown inFIGS. 56-59, and the optical connection module MG1 that causes coupledlight between the surface-emission laser diode chip and the 1st opticalfiber group. Yet the concept of this invention is not only applied tooptical connection module MG1 that couples optically thesurface-emission laser diode chip and the first optical fiber group MFG1but it is also applied to the optical connection module MG2 so thatconnects the first optical fiber group MFG1 and the second optical fibergroup MFG2 optically.

[0658]FIG. 60 shows an example of optical connection module MG2 thatcouples optically such an optical fiber group to a different opticalfiber group.

[0659]FIG. 60 is referred to.

[0660] In this case, the means for discriminating the direction isprovided on the fiber holders 64 and 65 in the form of arrow mark suchthat that each optical fiber fg1 of the first optical fiber group MFG1couples properly to the corresponding optical fiber fg2 in the secondoptical fiber group MFG2. By providing such discrimination means itbecomes possible to discriminate the a mutual direction instantaneouslyat the time of the connection between the first optical fiber group MFG1and the second optical fiber group MFG2, as the opticaltransmission/reception system is constructed in accordance with thepresent invention.

[0661] Furthermore, FIGS. 39-59 are examples of the discrimination meansand the present invention is not limited to such an arrow mark. Thediscrimination means might well use the difference of color. Thediscrimination means might well be able to use the tactile sense thatsenses the unevenness and others of a certain form. By using thediscrimination means that is able to use the tactile sense, theadvantage is being able to feel the direction of the fiber holder in thecase of working during darkness, or even in the case of performingconstruction work at night.

[0662] In the construction of FIG. 60, not only the discrimination meansis provided to each of the first fiber group holder 64 and the secondfiber group holder 65, but there are formed the flange surfaces on theedge part of the first fiber group holder 64 (A part of FIG. 60) and onthe edge of the second fiber group holder 65 (B part of FIG. 60) suchthat the flange surfaces engage with each other in the state that thereis an optimum optical coupling.

[0663] Furthermore, such discrimination means and thelocation/connection means can be used also in the optical connectionmodule MG3 that couples the second optical fiber group MFG2 and thethird optical fiber group MFG3 optically or in the optical connectionmodule MG4 that couples optically the third optical fiber group MFG3 andthe photodiode chip 34. When the optical transmission/reception systemis constructed according to this invention, the mutual orientation canbe discriminated between the optical fiber groups or between the opticalfiber group and the photodiode chip instantaneously and exactpositioning and optical coupling is achieved.

[0664] [Ninth Embodiment]

[0665] Furthermore, the optical connection module according to anotherembodiment of the present invention is accomplished by using pluraloptical fibers in parallel. It should be noted that the reason it hasbecome possible to construct the large-capacity opticaltransmission/reception system for a distance from several centimeters toseveral hundred kilometers is that stable laser oscillation ofsurface-emission laser diode at the wavelength of 1.1-1.7 μm wavelengthhas become possible as a result of the present invention as notedbefore. By using the surface-emission laser diode of the 1.1-1.7 μmoscillation wavelength, the inspection of laser elements in the laserdiode array is substantially facilitated. Further, the productivity ofthe optical transmission/reception system is improved remarkably.Construction of such an optical transmission/reception system was notpossible in the conventional surface emission laser diode of 0.85 μmwavelength band. By using the surface-emission laser diode the presentinvention, it became for the first time possible to realize a commercialbase optical transmission/reception system.

[0666]FIG. 61 shows another example of the telecommunication system thatuses a long-wavelength surface-emission laser diode. It should be notedthat FIG. 61 shows an optical fiber 72 drawn out from a module package71 in which a surface-emission laser diode chip is accommodated and anoptical fiber 73 for telecommunication that is connected to the opticalfiber 72. In FIG. 61, those parts explained previously are designated bythe sane reference numerals and explanation is omitted.

[0667]FIG. 61 is referred to.

[0668] The optical fiber 72 drawn out from module package 71 via theconnector 71A is connected by welding or other methods to the opticalfiber 73 for telecommunication at fiber connection part 74. In thiscase, a predetermined connection margin is required. With regard to theoptical fiber cable 72 drawn out from the module package 71, when theoptical fiber length Lg shown in FIG. 61 is too short, a very fine workis needed to assemble the module package 71. Thereby, the productioncost is increased. Also it becomes difficult to produce a high-precisionmodule.

[0669] The optical fiber used in the telecommunication systems of thisinvention is a very minute thing. The diameter is only a 50 μm or 62.5μm in the typical case. When assembling the module package 71 by usingsuch an optical fiber, for example, it is unnecessary to hold theoptical fiber 72 extending from the module package 71 by using a pair oftweezers. Thereby, it is difficult to hold the optical fiber 72 unlessthe length Lg of the optical fiber 72 is less than a certain length.

[0670] Much of the assembling work can be done by an automationapparatus. Even in such a case, a tool is necessary for holding such aminute optical fiber. In consideration of easiness of holding the distalend part of the optical fiber by the holding tool, it is necessary tosecure a certain length Lg for the optical fiber 72.

[0671] The inventor of the present invention, recognizing the importanceof doing the assembly work smoothly, conducted and evaluated extensiveassembly of optical fiber systems. Through various kinds of experimentalproduction operations, it was determined that length Lg shown in FIG. 61must be at least 1 mm. In other words, the length Lg (the guide opticalfiber length Lg shown in FIG. 61) of the optical fiber 72 drawn out fromthe module package 71, in which the surface-emission laser diode chip isaccommodated, has to be set to one or more millimeters.

[0672] In the case the length of this part is reduced to the order ofmicrons comparable to the optical fiber diameter, assembling work has tobe done under a microscope, using equipment that is expensive andhigh-precision and highly mechanized. Such work is unrealistic fromindustrial view point.

[0673] It should be noted that such a guide optical fiber 72 isconnected to the optical fiber constituting a part of the opticaltelecommunication system shown in FIG. 61 by welding. Thereby, there isa need to secure a welding margin (margin Gm shown in FIG. 62) in viewof melting of the fiber edge of the guide optical fiber 72. The weldingmargin would be the order of millimeters.

[0674] The inventor of the present invention conducted weldingexperiments and evaluated the necessary length of the welding margin Gmof FIG. 62.

[0675] In the case that guide optical fiber length Lg is in the order ofmicrons, it was discovered that the fiber edge of the optical fiber 71melts excessively. For example, in the case the length Lg is 200 μm, 500μm or 900 μm, the edge surface of optical fiber 71 melted excessivelyand a fine joint was not obtained. On the other hand, it was confirmedthat a fine joint can be obtained in the case the length Lg is made tobe one millimeter or more, such as 1 mm or 3 mm.

[0676] In summary, it is concluded that assembling of then modulepackage 71 requires that the length Lg be one or more millimeters inorder to connect with the optical fiber of the optical telecommunicationsystem to the guide optical fiber.

[0677] With regard to the upper limit of the guide optical fiber 72, itis sufficient for module assembly work to secure the length of 20 mm forthe guiding purpose. In the case the optical fiber 7 is too long, theexcessive optical fiber can be cut after the assembling of the opticalmodule 71 to a suitable length.

[0678] Next is an explanation about another aspect of this embodiment.

[0679] The above explanation discussed the relation with thesurface-emission laser diode 32 acting as the optical source and theguide optical fiber 72, while the same relation holds also at thedetecting side.

[0680] While illustration is omitted, it is possible to realize aphotodetection unit by replacing the surface emission-laser diode 32 ofFIGS. 61 and 62 with the photodetection device 34 such as photodiode.Thereby, there is an optical fiber corresponding to the guide opticalfiber 72 of FIGS. 61 and 62 in direct contact with the photodetectiondevice 34.

[0681] The same thought is necessary with regard to the moduleassembling and connection to the optical telecommunication system in theside of the photodetection device.

[0682] In the present invention, a highly reliable and practicalassembling of the photodetection unit is achieved by setting the lengthof the guide optical fiber connected directly with the photodetectiondevice 34 (corresponding to the guide optical fiber 72 of FIGS. 61 and62) to one millimeter or more. Further, a reliable connection isachieved with the optical fiber of the optical telecommunication system.

[0683] With regard to the upper limit, a length of 20 mm is sufficientsimilarly to the module package for the optical source side.

[0684] [Tenth Embodiment]

[0685]FIG. 63 shows the example of a telecommunication system that usesthe long-wavelength surface-emission laser diode chip 32 with the laseroscillation wavelength 1.1-1.7 μm band with plural optical fibers. Thoseparts explained previously in the FIG. 63 are designated with the samereference numerals and explanation is omitted.

[0686] In the construction of FIG. 63, the laser diode 32A in the laserdiode chip 32 is driven by a communication control unit 81 via a laserdiode driver circuit 82. The optical signal is supplied to the opticalfibers 72 and 73 in the form of an optical beam.

[0687] Conventionally, an optical telecommunication system was studiedby using the laser oscillation wavelength of 0.85 μm. However, becauseof the large transmission loss in the optical fiber, it was notpractical for long distance telecommunication. Further, in the case ofthe telecommunication system that uses the plural optical fibers incombination of conventional edge-emission type laser diodes, there is aneed of adjusting the optical coupling for each the optical fibers witheach of the laser diodes. Thus, the adjustment process was complicated.Further, it was difficult to connect the laser diodes directly to form atwo dimensional array. Further, in an edge-emission laser diode, theemission angle of the laser beam is large and the laser beam generallyhas a large aspect ratio. Thus, it has been provide a coupling lensbetween the emission part and the optical fiber for better opticalcoupling efficiency.

[0688] Contrary to the prior art, a stable drive is realized with lowenergy and low heating by using the long-wavelength surface-emissionlaser diode of 1.1-1.7 μm band according to this invention.

[0689]FIG. 64 shows the construction of the surface-emission laser diodeof the 1.3 μm band according to this invention. The emission angle ofthe laser beam in such a surface-emission laser diode is smaller ascompared with an edge-emission laser diode and takes the value of about15 degrees in both the vertical and horizontal directions. The laserbeam form is circular, and there is no need of shaping the laser beam.

[0690] Thus, individual laser diodes 32A can be connected withcorresponding optical fibers 72 without using a coupling lens, as longas the irradiation diameter of the laser diode is smaller than the corediameter of the optical fiber 72. The same is true also in otherlong-wavelength bands.

[0691] Thus, by using a surface-emission laser diode according to thepresent invention, the difference between the irradiation diameter ofthe laser diode 32A and the optical fiber core diameter provides atolerable margin, and it becomes possible to adjust plural opticalfibers altogether.

[0692] As shown in FIG. 64, the laser beam that is emitted from anemission part, in other words, the laser diode 32A, is injected withhigh efficiency into the core at the edge surface of the optical fiber.The optical signal thus injected into the optical fiber is transmittedover a long distance with little transmission loss due to the long thewavelength of the optical signal. By using plural optical fibers, a veryhigh quality optical telecommunication system of practical value ismaterialized. The laser diodes 32A can form in arbitrary two-dimensionalarrangement in the surface-emission laser diode chip 32 as long as theydo not cause interference.

[0693] The plural optical fibers 72 are fixed with each other by using ajig 91 shown in FIGS. 65A-65C, in which each of the optical fibers isprovisionally fixed by the jig 91 and pouring a resin 92 into the jig asrepresented in FIG. 65C. Thereby, the positional adjustment can beomitted at the time of system construction, by providing a positioningguide in the connector connection part 71A. Thus, it becomes possible toeasily construct an optical telecommunication system while using pluraloptical fibers.

[0694] Meanwhile, the number of the optical fibers used in an opticaltelecommunication system changes depending on the system. Thus, it isadvantageous to design the system such that the number of optical fiberscan be changed flexibly.

[0695] Thus, as shown in FIG. 66B, a number of openings having the sizeof the optical fiber diameter are formed on the connector connectionpart 71 a, and the connector connection part 71 a is attached on thesurface-emission laser diode chip 32 of FIG. 49C with an adjustment.Further, several kinds of connectors 71 b carrying plural optical fibersas shown in FIG. 66A are provided, and one of the connectors 71 b isselected and inserted into the connector connection part 71 a. By usingsuch a construction, waste of optical fibers in the optical connector71A is eliminated, and it becomes also possible to add optical fiberslater according to the needs. Thereby, it becomes possible to increasethe degree of freedom of design of the optical telecommunication system.

[0696] Furthermore, in long-wavelength surface-emission laser diode chip32 of 1.1-1.7 μm band of this invention, the laser diode elementsconstituting the emission part 32A can be arranged in two-dimensionalarray. In view of this, each optical fiber 95 is provided with a resincoating 96 having a hexagonal cross section as shown in FIG. 67 suchthat the two-dimensionally arranged optical fibers do not contactdirectly with each other. Thereby, an optical cable having a closestpacking structure for the optical fibers is realized as represented inFIG. 68. In the optical fiber cable of FIG. 68, the cross-sectional areaof the optical fibers is minimized because of the closest packingarrangement of the optical fibers.

[0697] In the construction of FIG. 67, it is also possible to use afixing member of a resin having a number of holes of hexagonalarrangement for holding the optical fibers, in place of providing aresin coating 96 outside the optical fiber 95.

[0698] [Eleventh Embodiment]

[0699]FIG. 69 shows an example of the optical fiber 101 used in atelecommunication system that uses a long-wavelength surface-emissionlaser diode. Those parts explained previously are designated by the samereference numerals and the description thereof will be omitted.

[0700]FIG. 69 is referred to. The optical fiber 101 is injected with thelaser beam emitted from the laser diode emission part 32A. The opticalfiber 101 consists of cladding 101B enclosing a core 101A, and the core101A transmits the laser beam thus injected. The diameter D of the core101A is determined with regard to the length L so as to satisfy thecondition 105≦L/D≦109.

[0701] Conventionally, an optical telecommunication system was studiedby using the laser oscillation wavelength of the 0.85 μm band. However,the transmission loss of the optical fiber was too large and there wasno practical value. Also there was no laser diode that can oscillatestably in the long-wavelength band, in which the transmission loss isminimum, was not realized. In accordance with the present invention, asurface-emission laser diode oscillating stably at the laser oscillationwavelength of 1.1-1.7 μm band is materialized as a result of improvementof the semiconductor Bragg reflectors 12 and 18 and by the provision ofthe non-optical recombination elimination layers 13 and 17. As a result,a practical long wavelength band optical telecommunication system becamepossible.

[0702]FIG. 70 shows the construction of a long distance opticaltelecommunication system using the above long-wavelengthsurface-emission laser diode chip 32 and the optical fiber 101.

[0703]FIG. 70 is referred to. The long distance opticaltelecommunication system uses the long-wavelength surface-emission laserdiode chip 32. The optical fiber is injected with the laser beam that isemitted from the laser diode emission part 32A of the laser diode chip32. Further, the optical telecommunication system includes a firstoptical fiber 101A having the core diameter D of 50 μm and the length Lof 5 km and second optical fiber having the core diameter D of 50 μm andthe length L of 5 km injected with the laser beam emitted from theoptical fiber 101A, wherein the optical telecommunication system furtherincludes the photodiode chip 34 including the photodetector part 34Ainjected with the laser beam emitted from the second optical fiberBetween the laser diode chip 32 and the first optical fiber 101 A, theconnection module 71 is provided, and a similar connection module 75 isprovided between the second optical fiber 101B and the photodiode chip34. Furthermore a repeater 101C is provided between the first opticalfiber 101A and the second optical fiber 101B for amplification and therepeating of the optical signal.

[0704] In the constitution of FIG. 70, it should be noted that the firstand second optical fibers 101A and 101B are produced usually withseveral thousand kilometers as a unit. However, a length of severalhundred kilometers becomes the unit of the optical fiber product due toexistence of pores or other defects that occur during the productionprocess. In consideration of transmission loss, the practical length ofthe optical fiber to the repeater 101C is 5-50 km in the case theoptical fiber has a core diameter of 50 μm. The optical fiber of thislength is used for the optical fiber telecommunication.

[0705] In the case the transmission length is longer than 50 km, it isnecessary to provide a relay point for signal amplification in view ofthe transmission loss. When the distance is shorter than 5 m, varioustelecommunication means are available other than the opticaltelecommunication system.

[0706] For optical fiber 101 used with this embodiment, a quartz glassoptical fiber having a core diameter D of several microns, or a plasticsoptical fiber having a core diameter D of several hundreds microns maybe used. These optical fibers can be used alone or in the form of abundle.

[0707]FIG. 71 shows an optical telecommunication system forbi-directional optical telecommunication by using a bundle of quartzglass optical fibers 101-A and 101-B each having a diameter D of 50 μmand a transmission length L of 500 m, wherein these optical fibers areseparated from each other at the photodetection end and the opticalsource end.

[0708]FIG. 71 is referred to.

[0709] There can be seen that optical transmission/reception parts 102Aand 102B are provided in the present invention in which thelong-wavelength surface-emission laser diode chip 32 and the photodiodechip 34 form a pair. Thus, the first optical fiber 101A is coupled tothe emission part 32A of laser diode chip 32 in the opticaltransmission/reception part 102A, and the optical fiber 101 A is furthercoupled optically to the optical detection part 34 A of photodiode chip34 in the optical transmission/reception part 102. Similarly the secondoptical fiber 101B is coupled optically to the emission part 32A of thelaser diode chip 32 in the optical transmission/reception part 102B, andthe optical fiber 101B is coupled optically to the optical detectionpart 34A of photodiode chip 34 in the optical transmission/receptionpart 102A. Thereby, the laser diode chip 32 and the photodiode chip 34constituting the optical transmission/reception part 102A form theconnection module 71. Also, the laser diode chip 32 and the photodiodechip 34 constituting the optical transmission/reception part 102B formthe connection module 75.

[0710] By expanding the foregoing construction of using thelong-wavelength surface-emission laser diode chip 32 and thephotodetector chip 34 as a pair for the case in which a number of laserdiode elements 32A are formed on the laser diode chip 32 and a number ofphotodiode elements are formed on the photodetector chip 34, it ispossible to realize a large capacity optical telecommunication system.

[0711]FIG. 72 shows the case of applying the present invention to a highspeed multimedia network that uses a fluorinated plastic optical fiberhaving a core diameter of 100 μm and the transmission length of lessthan 100 m.

[0712]FIG. 72 is referred to.

[0713] A first optical fiber 111 having a core diameter D of 50 μmextends from a station-side apparatus 110 and a second optical fiber112, formed by bundling 50 optical fibers each having a core diameter Dof 50 μm and a length of 1 km, is connected to the optical fiber 111. Asa result, there is formed a the high speed optical transmission systemcapable of transmitting information with the speed of the order of Gbpswith a total distance of 100 km. In the example of FIG. 72, the opticalrepeater 111R is provided between the optical fiber 111 and opticalfiber 112, to compensate for the damping of the signal by thepropagation loss of quartz glass optical fiber.

[0714] The optical signal transmitted through the second the opticalfiber 112 enters into a network terminator 115 at first and converted toan electric signal. Further, the electric signal is distributed tonecessary lines 115 a-115 c, and the electric signal thus supplied toeach line is transmitted further in the form of the optical signal by anoptical telecommunication system 116 that includes the opticaltransmission/reception parts 102A and 102B explained to with referenceto FIG. 71. The optical signal thus transmitted is supplied to acorresponding optical output port. In the optical telecommunicationsystem 116, an optical fiber 117 a having a core diameter of 100 μm anda length of 10 m is used for the optical transmission to the opticaloutput port corresponding to the line 115 a, while an optical fiber 117b having a core diameter of 100 μm and a length of 50 m is used for theoptical transmission to the optical output port corresponding to theline 115 b. Further, an optical fiber 117 c having a core diameter of100 μm and a length of 100 m is used for optical transmission to theoptical output port corresponding to the line 115 c. The optical signalsthus transmitted are converted to electric signals by the opticaltransmission/reception part 102B in each optical output port and issupplied to corresponding terminal devices 102C.

[0715] Further, the transmission from each terminal device 102C istransmitted to the optical transmission/reception part 102 A in the formof an optical signal through the optical fibers 117 a-117 c and reachesthe station-side apparatus 110.

[0716] In this embodiment, the core diameter of the optical fibers 117a-117 c has a large value of 100 μm, and high-precision alignment byusing a high-precision lens system is not necessary. Thus, it becomespossible to achieve optical connection to various apparatuses in a homeor office quite extremely.

[0717] In the present embodiment, too, the relationship of 105≦L/D≦109is maintained between the length L and core diameter D of the opticalfibers. When the transmission distance is longer than 100 km, nosubstantial transmission is possible due to the transmission loss, andthus, it is necessary to provide a relay point for amplification andrepeating. In the case the transmission distance is shorter than 10 m,such an optical telecommunication system is not always necessary and maybe replaced with other means.

[0718]FIGS. 73A and 73B show the application of the long-wavelengthsurface-emission laser diode of the present invention to a hybridoptical integrated circuit device, respectively in a plane view and across-sectional view.

[0719]FIGS. 73A and 73B are referred to. The long-wavelengthsurface-emission laser diode chip 32 and the photodiode chip 34 arecarried on a ceramic substrate 51 in this embodiment. Further, theceramic substrate 51 carries thereon a first optical waveguide 52 havinga rectangular cross-section of 7×7 μm and a length of 1 cm for guidingthe laser beam emitted from the laser diode 32A. Further, the ceramicsubstrate 51 carries a second optical waveguide 53 having a rectangularcross-section of 7×7 μm and a length of 1 cm similar to the opticalwaveguide 52 in optical coupling with the photodetector part 34 A of thephotodiode chip 34 for supplying the optical signal to the photodetectorpart 34B. Further, a preamplifier 51 for amplifying the output electricsignal of the photodetector chip 34 is provided on the ceramic substrate51.

[0720] Further, an optical fiber 54 having a core diameter D of 100 μmand a length of 100 m is provided in optical coupling with the firstoptical waveguide 52 via a connection module 56, and a second opticalfiber 55 having a core diameter D of 100 μm and a length of 100 m isprovided in optical coupling with the second optical waveguide 53.

[0721] Further, a connection module 57 equipped with thesurface-emission laser diode chip 32 and the photodiode chip 34 isprovided at the other end of the optical fibers 54 and 55 such that theoptical fiber 54 is coupled with the photodetector part 34 A of thephotodiode chip 34 and the optical fiber 55 is coupled with the laserdiode 32 A of the laser diode chip 32.

[0722] The optical waveguides 53 and 54 are formed on a siliconsubstrate 58 by the steps of forming the respective cores and thecladdings that cover the cores by a photolithographic process, and thesilicon substrate 58 is provided on the ceramic substrate 51 commonlywith the preamplifier 51A.

[0723] [Twelfth Embodiment]

[0724]FIGS. 74A and 74B show an example of a laser diode chip 120 thatis used with the optical telecommunication system in which thelong-wavelength surface-emission laser diode of the present invention isused, wherein FIG. 74A shows a plane view while FIG. 74B shows across-sectional view taken along a line A-A′. It should be noted thatthe scale FIG. 74A is not identical with the scale of FIG. 74B. In thedrawings, those parts explained previously are designated by the samereference numerals and the description thereof will be omitted.

[0725]FIGS. 74A and 74B are referred to.

[0726] It can be seen that the long-wavelength surface-emission laserdiode 32A and the photodetection device 34A are formed to monolithicallyon a laser diode chip 120.

[0727] The photodetection device 34A has a stacked structure similar tothe stacked structure of the long-wavelength surface-emission laserdiode 32A and is formed simultaneously with the process of forming thesurface-emission laser diode 32A. The structure thus formed can be usedas a photodetection device, by applying with a reverse bias or usingwithout a bias. This photodetection device 34A has sensitivity to theoscillation wavelength of the surface-emission laser diode 32A and candetect the optical emission of the laser diode 32A.

[0728] As will be understood from the plane view of FIG. 74A, thephotodetection device 34A is formed in such a form to surround thelong-wavelength surface-emission laser diode 32A. On the top surface ofthe laser diode 32A, an upper electrode 121 is formed and a lowerelectrode 123 is formed on the bottom surface of the laser diode chip120. Also another upper part electrode 122 is formed on the top surfaceof the photodetection device 34A. An optical window is formed in theupper part electrode 121 of the laser diode 32A for taking out theoptical output. Such a window is not formed in upper part electrode 122the of photodetection device 34A.

[0729]FIG. 75 is the figure that explains the operation of the laserdiode chip 120 shown in FIG. 74.

[0730]FIG. 75 is referred to.

[0731] The laser diode 32A in the surface-emission laser diode chip 120is positioned so as to face the end surface of an he optical fiber 125,and the optical beam emitted from the surface-emission laser diode 32Ais injected into the core of the optical fiber. Thereby, there occurs aleakage light in the lateral direction from the sidewall of the,surface-emission laser diode 32A, and the leakage light is detected bythe photodetection device 34A provided adjacent to the laser diode.While the amount of the leakage light from the sidewall of thesurface-emission laser diode 32A is not much, nevertheless the leakagelight is detected by the photodetection device 34A, as thephotodetection device 34A is formed so as to surround thesurface-emission laser diode 32 A in the construction of FIG. 75.

[0732] Furthermore, it should be noted that the semiconductor Braggreflector 18 and the upper part electrode 122 are formed on thephotodetection device 34 A in FIG. 75. Therefore, the optical signal andthe like transmitted from another party via the optical fiber 125 isreflected at the surface of the photodetection device 34A as shown withthe arrow in the drawing. Thereby, the photodetector 34A does not detectsuch an optical signal.

[0733] As it will be clear from the aforementioned explanation, thepresent invention forms the photodetection device for detecting theoutput detection of the surface-emission laser diode integrally on thesurface-emission laser diode chip. By integrating such a laser diodechip on a ceramic substrate 111, it becomes possible to construct theoptical telecommunication system having a hybrid constitution.

[0734]FIGS. 76A and 76B show an example of the semiconductor laser diodechip used with the optical telecommunication system in which thelong-wavelength surface-emission laser diode of this invention is used,wherein FIG. 76A shows a plane view while FIG. 76B shows across-sectional view that taken along a line A-A′. The scale of FIG. 76Aand FIG. 76B are not the same.

[0735]FIGS. 76A and 76B are referred to.

[0736] The long-wavelength surface-emission laser diode 32A and thephotodetection device 34A are integrated monolithically on a laser diodechip, wherein the photodetection device 34A has a stacked structuresimilar to the stacked structure of the long-wavelength surface-emissionlaser diode 32A.

[0737] Thus, the photodetection device 34A is formed simultaneously withthe same process as the one used for forming the surface-emission laserdiode 32A. The photodetection device 34A thus formed has sensitivity tothe wavelength of the surface-emission laser diode 32A and can detectthe optical radiation produced by the laser diode 32A.

[0738] In the present embodiment, the upper semiconductor Braggreflector 18 is removed from the photodetection device 34A by an etchingprocess. As can be seen from the plane view of FIG. 76A, thephotodetection device 34A is formed so as to surround thelong-wavelength surface-emission laser diode 32A, and the devices 32Aand 34A have respective top surfaces provided with the upper electrode121 and the upper electrode 122 formed with the window for opticalinput/output. Further, the bottom electrode 123 is formed on the bottomsurface of laser diode chip 120.

[0739]FIG. 77 explains the operation of the laser diode chip of FIG. 76.

[0740]FIG. 77 is referred to.

[0741] In the surface-emission laser diode chip 120, the laser diode 32Ais positioned so as to face the end surface of the optical fiber 125such that the optical beam emitted from the surface-emission laser diode32A enters into the core of optical fiber 125.

[0742] In this embodiment, the sidewall of the surface-emission laserdiode 32A and the sidewall of the photodetection device 34A are coveredwith the upper part electrode 121 or 122, and the leakage light is notformed at the side wall of the laser diode.

[0743] Thereupon, when an optical beam carryhinig an optical signalcomes in from another party on the line, the optical beam emitted fromthe end surface of the optical fiber 125 enters into the top surface ofthe photodetection device 34A while causing spreading as represented byan arrow in the drawing. The optical beam is then detected by thephotodetection device 34A formed so as to surround the surface-emissionlaser diode 32A. On the other hand, the top surface of surface-emissionlaser diode 32A is covered with the semiconductor Bragg reflector 18,and because of this, no substantial optical beam invades inside thesurface-emission laser diode 32A from outside.

[0744] Thus, as will be clear from the aforementioned explanation, itbecomes possible to construct an optical telecommunication system havingan optical transmission/reception unit of hybrid construction in thepresent embodiment, by forming the photodetection device 34A formonitoring the output of the surface-emission laser diode 32A integrallyon the surface-emission laser diode chip 120. Such a laser diode chip120 can be integrated on the ceramic substrate 111.

[0745] Furthermore the combination of the surface-emission laser diodeand the photodetection device that explained above is merely for thepurpose showing an example, and the present invention also includes thecase of forming an array of these devices or the case of combining themonitoring photodetector 34A for monitoring the output of thesurface-emission laser diode 32A and the photodetector 34A for detectingthe incoming signals are combined. Naturally, the present invention isnot limited to the specific mutual positional relationship or shape ofthe surface-emission laser diode and the photodetector.

[0746] [Thirteenth Embodiment]

[0747] Next, this invention will be explained about other examples.

[0748] It is important to use a highly strained layer of GaInAs, GaInNAsor GaAsSb for the active layer in order to realize the long wavelengthsurface emission laser diode of 1.1-1.7 μm. In order to achieve this, itis necessary to minimize the mechanical stress applied to the laserdiode. Such a mechanical stress may result from the thermal stresscaused between the laser diode and the mount substrate as a result ofthe operational temperature environment or due to the heating of thelaser diode or driver circuits. Such a thermal stress is caused in astructure in which materials of different linear thermal expansioncoefficients are fixed, due to the tendency of the structure that tendsto maintain original shape. Thus, the thermal stress depends on thetemperature change, the coefficient of linear thermal expansion, Youngmodulus, and the like. It is possible to eliminate the thermal stress bycontrolling the temperature of the whole module including the laserdiode. However, such a measure of providing a temperature regulatingmechanism is difficult in view of the increased cost. Further, it isdifficult to control the temperature completely constant.

[0749] Thus, it is desirable, in order to provide a highly reliablesystem with low cost, to use a material having a coefficient of linearthermal expansion close to that of the laser diode for the mountsubstrate so as to reduce the influence of the thermal stress on thelaser diode as small as possible.

[0750] In this embodiment, various mount substrates were produced byusing various materials having different coefficients of linear thermalexpansion and evaluation was made about the influence of the thermalstress on the output characteristic of the laser at the time of thelaser oscillation. The surface-emission laser diode used in theexperiment has the structure explained with reference to FIG. 1 and thelaser oscillation wavelength was 1.3 μm. Further, a chip size of 5 mm×10mm (thickness 0.6 mm) was used in which 20 laser diodes were alignedwith a pitch of 300 μm. The size of the mount substrate made was 10mm×20 mm (thickness 2 mm).

[0751] The result is shown in Table 5. In Table 5, ◯ represents thesample in which a stable output was obtained in the operationalenvironment of 0-70° C. while X represents the sample in which no stableoutput was obtained and hence not suitable for practical use. TABLE 5linear thermal expansion material coefficient laser output quartz glass0.3 × 10⁻⁶/K X smicrystal   2 × 10⁻⁶/K X CVD diamond   2 × 10⁻⁶/K X Si  4 × 10⁻⁶/K ◯ SiC   4 × 10⁻⁶/K ◯ AlN   5 × 10⁻⁶/K ◯ GaAs   6 × 10⁻⁶/K ◯Al—Si(60Al-  15 × 10⁻⁶/K X 40Si) Cu  17 × 10⁻⁶/K X

[0752] The coefficient of linear thermal expansion of the laser diode ofthis invention is 6×10⁻⁶/K. From the above results, it can be seen thatthe thermal stress is reduced and a stable laser output is obtained whenthe difference of the linear thermal expansion coefficient between thelaser diode and the mount substrate within about 2×10⁻⁶/K. Particularly,the materials such as Si, SiC, GaAs, AlN are easily available and areeasily processed and handled for the mount substrate.

[0753] Further, by choosing the heat radiating member on which the mountsubstrate is mounted such that the material of the heat radiating memberhas a linear thermal expansion coefficient close to that of the laserdiode, the stress to the mount substrate is minimized and the mechanicalstress applied to the laser diode is reduced as a result. Further, thematerial constituting the heat radiation member is required to have ahigh thermal conductivity.

[0754] In the present invention, various heat radiating members areformed by using various materials having different linear thermalexpansion coefficients and the effect of heat generated at the time ofthe laser oscillation on the laser output characteristics is evaluated.

[0755] The laser diode used for the evaluation was the same as the oneused in the evaluation of Table 5. In this experiment, an SiC substratehaving the same size as in the previous experiments was used for themount substrate.

[0756] Table 6 shows the results, wherein ◯ in Table 4 represents thesample in which a stable output was obtained in the operationalenvironment of 0-70° C., while X represents the case in which no stableoutput was obtained. TABLE 6 material thermal conductivity laser outputSiO2 about 8 W/mK X Al2O3 about 17 W/mK X Cobar about 17 W/mK X AlNabout 200 W/mK ◯ Cu/W 180-200 W/mK ◯ W about 170 W/mK ◯ Mo about 160W/mK ◯ Cu about 390 W/mK ◯

[0757] The thermal conductivity of the laser diode of this invention is55 W/mK. Thus, from the results noted above, it can be seen that apreferable result is obtained when the thermal conductivity of the heatradiating member is larger than the thermal conductivity of the laserdiode. Thus, the heat generated at the time of the laser oscillation istransferred to the mount substrate and then to the heat radiation memberwithout returning to the laser diode when the thermal conductivity ofheat radiating member is larger than the thermal conductivity of thelaser diode of this invention. Thus, the output fluctuation of the laseraccompanied by the effect of heat accumulation does not result and astable characteristic suitable for practical use is obtained.Particularly, the materials such as AlN, Cu/W, W, Mo, Cu are easilyavailable and can be processed easily to form a heat radiating member.

[0758] Especially the use of Cu/W is advantageous as it allows controlof the thermal conductivity by controlling the compositional ratio andcan be used as a he package substrate to be explained later withreference to FIG. 79.

[0759] Hereinafter, an optical telecommunication system of the presentembodiment that uses such a member will be explained.

[0760] The telecommunication system is composed of the optical fiber oroptical waveguide that operates as the transmission path between thephotodetection part including a surface photodetection device and adriver circuit thereof and the optical transmission part including asurface-emission laser diode and a driver circuit.

[0761] The driver circuit of the laser diode or the photodetector isformed on the same mount substrate commonly with the laser diode of thephotodetector to which the drive circuit cooperates, Alternatively, thedriver circuit is formed on the laser diode substrate by a wafer processsimultaneously to the process of forming the respective devices to whichthe driver circuit cooperates. Further, the optical telecommunicationsystem performs the bi-direction telecommunication by providing theoptical transmission part and the photodetection part at both ends ofthe optical transmission path.

[0762]FIG. 78 shows an example of the optical transmission part of suchan optical telecommunication system. In the drawing, those partsexplained previously are designated by the same reference numerals andthe description thereof will be omitted.

[0763]FIG. 78 is referred to.

[0764] The optical transmission part includes the surface-emission laserdiode chip 32 and a driver circuit 32DR driving the laser diode chip 32,a mount substrate 131 on which the laser diode chip 32 and the drivercircuit 32DR are mounted, a heat radiation member 132 adjustablysupporting the mount substrate 131 and for radiation of the heat, and ametal package 133 holding the heat radiation member 132 and acting asthe heat sink or heat radiation fin, and an optical transmission 134acting as the optical path. The metal package 133 and heat radiationmember 132 and also mount substrate 131 are connected mechanically andthermally with each other by a solder or a resin. Also, the laser diode32 and the driver circuit 32DR are connected electrically by wirebonding, and the like.

[0765] For example, the laser diode chip 32 includes the laser diode ofFIG. 1, wherein the laser diode may be the one that oscillates at thewavelength of 1.2 μm. As for the mount substrate 131, on the other hand,a Si substrate having the the coefficient of linear thermal expansion of4×10⁻⁶/R can be used. In this case, the laser diode 32 is die-bondedwith an AuSn solder on the mount substrate 131. Thereby, electrode andelectric mechanical connection are achieved.

[0766] It should be noted that an SiO₂ film 200 nm thickness is formedon the surface of the Si substrate 131. This SiO₂ film may be formed bya thermal oxidation process or a CVD process or an SOG process. Theforegoing oxide film is used for insulation, but the heat radiationcharacteristic of such a film is inferior to Si. Therefore, it ispreferable that the insulation film has as small thickness as possiblewithin the range of providing sufficient electric insulation. The oxidefilm may be omitted if it is possible The driver circuit 32DR thatdrives the laser diode chip 32 is mounted on the same mount substrate131 together with the laser diode chip 32. A thermal conductive AlNsubstrate (coefficient of linear thermal expansion 5×10⁻⁶/K, thermalconductivity 200 W/mK) may be used as the heat radiating substrate thatcarries the mount substrate. The AlN substrate has excellentcompatibility with regard to the coefficient of linear thermalexpansion. Further, a powder mold product of Cu/W may be used for themetal package 133. For example, the molded of Cu/w product forming themetal package 133 may have the composition of 89W-11Cu and a linearthermal expansion coefficient of 6.5×10⁻⁶/K and a thermal conductivityof 180 W/mK. By using such a powder mold product, a highly precisionproduct is obtained with low cost. Further, it is possible to form heatradiation fins easily and efficient heat radiation becomes possible.

[0767] In the example of FIG. 7B, a multiple mode optical fiber having acore diameter of 50 μm is connected optically as the optical fiber 134with the laser diode 32A in the laser diode chip 32. The stability ofsuch an optical telecommunication system was evaluated in theoperational environment of 0-70° C. from and it was confirmed that thelaser output is stable. No change of output characteristics GaAsobserved. Further, there was no lifetime degradation. Thus, it was foundthat an excellent optical telecommunication system can be constructed.

[0768] Furthermore, It is possible to use a GaAs (coefficient of linearthermal expansion 6×10⁻⁶/K) or AlN (coefficient of linear thermalexpansion 5×10⁻⁶/K) or SiC (coefficient of linear thermal expansion4×10⁻⁶/K) for the substrate as the mount substrate 133 instead of the Sisubstrate. Because these substrates are an insulating substrate,formation of oxide film is not necessary when these substrates are used.Otherwise, the same structure may be used. In view of the cost andeasiness of handling, the use of AlN is most preferable.

[0769] Also, good results were obtained when Cu/W (coefficient of linearthermal expansion is 6-8×10⁻⁶/K, thermal conductivity is 180-200 W/mK)such as 89W-11Cu, 85W-15Cu or 80W-20Cu, or the metals of W, Ma, Cu areused for the heat radiation member in place of AlN.

[0770] In the example of FIG. 78, it was explained based on theconstruction that there is only one laser diode in the laser diode chip32 forming the transmission part. However, it is possible to form anarray of plural laser diodes also in the laser diode chip 32 or, byforming the array of the laser diode chip 32. Thereby, a large capacityoptical telecommunication system is constructed.

[0771] For example, in the case of the multiple laser array chip inwhich plural laser diodes 32A are formed on a single chip 32, the laserdiodes 32A come close with each other and the fluctuation of the laseroutput characteristic may be influenced by the thermal stress caused bythe heat generation and accumulation of the heat thus generated.Furthermore the laser diode driver circuit 32DR is often formed also onsuch a chip. Thus, the heat from such a driver circuit 32DR issuperimposed. In this embodiment, the laser output that stabilizedwithout bringing about such a problem by choosing the material of themount substrate 131 or the heat generation part 132 appropriately.

[0772] Formerly, the surface-emission laser diode having the oscillationwavelength of 1.1-1.7 microns did not exist. Therefore, thetechnological problems with regard to the optical telecommunicationsystem or mounting of the surface-emission laser diode chip of such along-wavelength band have not been announced. This time, the inventor ofthis invention recognized the concrete technological problems for thefirst time by this invention and provided the resolution.

[0773] In the explanation above, a multimode optical fiber was used forthe optical transmission path coupled optically to the laser diode chip32. However, the optical transmission path may also be a single modeoptical fiber or a plastic optical fiber. The laser diode of the presentinvention does not need expensive cooling device such as a Peltierdevice. But the present invention does not exclude the use do such acooling device.

[0774] Next, another embodiment of the present invention will beexplained with reference to FIG. 79, wherein those parts correspondingto the parts described previously are designated by the same referencenumerals and the description thereof will be omitted.

[0775] Referring to FIG. 62, the present embodiment also shows theoptical transmission part of the optical telecommunication system. Inthe present embodiment, the heat radiating member 132 functions also asthe metal package 133.

[0776] It should be noted that the illustrated optical transmission partincludes the laser diode chip 32 in which a number of laser diodes 32Aare arranged in the form of laser array, the driver circuit 32DR drivingthe laser diodes 32A, a mount substrate on which the laser diode chip 32and the driver circuit 32DR are mounted, a metal package 133 supportingthe mount substrate 131 and functioning as the heat sink and also a heatradiating fin, an optical fiber 134 constituting the opticaltransmission path, and a ferule fixing the optical fiber 134. A solderor a resin connects the metal package 133 and the mount substrate 131mechanically and thermally. Further, the laser diode chip 32 and thedriver circuit (not shown) are connected electrically by using a bondingwire.

[0777] In the illustrated example, the laser diode of FIG. 27 having theoscillation wavelength of 1.3 μm is used, and four devices are arrangedwith a pitch of 250 μm, which is the pitch identical with the pitch ofthe corresponding optical fibers. Similarly to the previous embodiments,a Si substrate was used for the mount substrate and the laser diode wasdie bonded on the mount substrate by using an AuSn solder. Further, theelectrodes are connected mechanically and electrically.

[0778] It should be noted that the surface of the Si substrate is formedwith an SiO2 film having a thickness of 200 nm by a thermal oxidationprocess. However, such an oxide film may be formed by a CVD process oran SOG process. Further, in view of the heat radiating performance ofthe SiO₂ film, it is preferable to form the SiO2 film with a thicknessas small as possible. The SiO2 film may be omitted if it is possible.Similarly, the driver circuit (not shon) that drives the laser diode isformed also on the common mount substrate.

[0779] As the metal package supporting the mount substrate and actingalso as the heat radiating member, a powder mold product of Cu/W isused. For example, the molded of Cu/W product forming the metal packagemay have the composition of 89W-11Cu and a linear thermal expansioncoefficient of 6.5×10⁻⁶/K, which is close to the linear thermalexpansion coefficient of the laser diode. Further, the metal package hasa thermal conductivity of 180 W/mK. By using such a powder mold product,a highly precision product is obtained with low cost. Further, it ispossible to form heat radiation fins easily and efficient heat radiationbecomes possible.

[0780] In the present embodiment, an array of multimode optical fibersin which the optical fibers each having a core diameter of 50 μm arearranged with a pitch of 250 μm is used. In the temperature range of0-70° C., there occurs no change of laser output and no lifetimedegradation. Thus, excellent optical telecommunication becomes possible.As the metal package functions also as the heat radiating member, thenumber of parts can be reduced and the efficiency of heat radiation ismaximized.

[0781] A similar result is obtained also when a GaAs substrate or AINsubstrate is used in place of the Si substrate In the case of the AlNsubstrate or SiC substrate, they are insulating substrates and theformation of the oxide film is not necessary. Otherwise, the foregoingdescription applies and further explanation will be omitted.

[0782] In the present embodiment, four laser diodes and four opticalfibers are used. However, the present invention is by no means limitedto this specific construction but the present invention is applicablealso to the case in which the number of the laser diode is one and thenumber of the optical fiber is one, or the number of the laser diodes is8, 12, 16 and there are 8, 12, 16 optical fibers in correspondencethereto.

[0783] In the case of the multi-laser diode array chip in which aplurality of laser diodes are formed on a common chip by exploiting thefeature of the surface-emission laser diode of the present invention, alarge number of laser diodes can be formed easily and is ideal for largecapacity telecommunication. On the other hand, in the case of such amulti-laser array chip, heat dissipation of the laser diodes becomes animportant issue. By using the material of the mount substrate and theheat radiating member as set forth in the present embodiment, it becomespossible to operate the laser diode array without causing problems.Thereby, a stable laser output can be obtained.

[0784] In the present embodiment, a multimode optical fiber was usedalso for the optical transmission path. However, this is not a mandatorycondition and it is possible to use an optical waveguide, single modeoptical fiber, plastic optical fiber, and the like.

[0785] [Fourteenth Embodiment]

[0786] Next a telecommunication system according to another embodimentof the present invention that uses the long-wavelength surface-emissionlaser diode of the present invention will be explained with reference toFIG. 80. In FIG. 80, those parts corresponding to the parts describedpreviously are designated by the same reference numerals and thedescription thereof will be omitted.

[0787] Referring to FIG. 80, the telecommunication system that uses thelong-wavelength surface-emission layer diode includes a laser diodemodule 141 carrying thereon the laser diode chip 32, a ferule 142Afixing the optical fiber 142, an adapter housing 143 accommodating theferule 142A together with the optical fiber 142, a bush 144 fixing theoptical fiber 142, a housing 145 accommodating the bush 144, and thebase 146 that holds the laser module 141 and the housing 143 as anintegral body. In the construction of FIG. 63, the length of the opticalfiber 142 from the point A to the point B is set longer than thedistance L (fixed distance) between the end surface of the adapterhousing 143 at the side of the laser diode chip 32 and the end surfaceof the housing 145.

[0788] More particularly, in the case the fixed length L is set to 1 inFIG. 80, the length A-B of the optical fiber 142 is set to 1.05.Thereby, the optical fiber 142 is provided with a bend, and theresilience associated with the bend causes the optical fiber 142 to beurged in the axial direction toward the optical emission part 32A (leftdirection in the drawing). Thereby, the mechanical connection isstabilized and an excellent optical coupling can be realized. Here, aplastic optical fiber is used as the optical fiber 142 but the presentinvention is by no means limited to such a specific case and it ispossible to use a quartz glass fiber having a plastic coating.

[0789]FIG. 81 shows the case in which the length L of FIG. 80 is set to1 and the length A-B of the optical fiber is set to 2. Other conditionsare identical with the case of FIG. 80. In the present embodiment, alarger resilient force is obtained as compared with the case of FIG. 80and the connection between the laser diode and the optical fiber becomesmore reliable.

[0790]FIG. 82 shows the cross-sectional view of the adapter housing 143used in the optical telecommunication system of FIG. 80 or 81.

[0791] Referring to FIG. 82, the adapter housing 143 includes a taperedsplit sleeve 1431, a ferule 142A fixing the optical fiber 142, a coilspring 1432 of a shape-memory alloy exerting a force in the axialdirection, a collar 1433 cooperating with the coil spring 1432 and aplug housing 1434 accommodating therein the split sleeve 1431, the coilspring 1432 and the collar 1433, and a connecting member 1435 forconnecting the plug housing 1434 to the adapter housing 143.

[0792] The coil spring 1432 exerts a resilient force at the ordinarytemperature or heated state determined by the initial shape. Thus, withcooling, the coil spring may extend and urge the ferule 142A in theaxial direction thereof. Depending on the design of the connectingmember 1435, it is possible to memorize the shape such the coil spring1432 extends at the high temperature side.

[0793]FIG. 83 shows a modification of FIG. 82, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

[0794] Referring to FIG. 83, the coil spring 1432 of the shape-memoryalloy is replaced with a leaf spring 1432A of a shape-memory alloy andthe leaf spring exerts a resilient force at the ordinary temperature orat the heated state determined by the initial shape of the spring. Thus,the leaf spring 1432A may extend with cooling and urge the optical fiber142 in the axial direction thereof. Depending on the design of theconnecting member 1435, the leaf spring 1432A may extend at the hightemperature side.

[0795] It should be noted that the spring 1432 or 1432A is not limitedto a shape-memory alloy but may be formed of a shape-memory plastic. Insuch a connection module, there exists a temperature difference betweenthe temperature in which the optical telecommunication system isassembled (generally higher than the room temperature) and thetemperature (room temperature) in which the system is used. Because ofthis temperature difference, the shape-memory material changes the shapethereof and generates a resilient force in the state that the member isassembled in a structural body. It should be noted that the sense oftemperature change (between the state in which the system is assembledand the state in which the system is used) may be reversed.

[0796]FIG. 84 shows another example of the telecommunication system thatuses the long-wavelength laser diode according to the present invention,wherein those parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

[0797] Referring to FIG. 84, the telecommunication system of the presentembodiment is formed of a laser module 141 carrying the laser diode chip32, a ferule 142A for fixing the optical fiber 142, an adapter housing143 accommodating the ferule 142A therein together with the opticalfiber 142, a fixing apparatus 147 holding and fixing a part of theoptical fiber 142, and abase 146 fixing the adapter housing 143 and thefixing apparatus 147 and the laser module 141 as an integral body.Thereby, the telecommunication system of FIG. 67 provides a bend to theoptical fiber 142 between the adapter housing 143 and the optical fiberfixing apparatus 147.

[0798] The optical fiber fixing apparatus 147 is a device that uses africtional grip compression and includes a shape-memory alloycompressional spring 1471, a collar 1472 cooperating with the spring1471, another collar 1473, a ball 1474 disposed between the collars 1472and 1473, and a housing 1475 having a conical surface in contact withthe ball 1474. In the housing 1475, the ferule 142B holding the opticalfiber 142 and the split sleeve 1476 are accommodated in the state thatthe shape-memory alloy spring 1471 intervenes between the collar 1472and the split sleeve 1476.

[0799] In the optical telecommunication system of such a construction,the fiber 142 urges a force acting in the left and right directions uponthermal deformation, while such an urging force pushes the ball 1474 inthe optical fiber fixing apparatus 147 in the right direction by theaction of the compressional spring 1471. Thus, the optical fiber 142cannot move in the right direction and exerts a force to the side of theadapter housing at the opposite side.

[0800] In the example of FIG. 63, the optical fiber 142 is provided witha bend and the optical fiber 142 urges the optical fiber 142 in thedirection of the laser diode chip 32 as a result of the resilient forceassociated with the bend of the optical fiber 142. In the example ofFIG. 84, the resilient force of the optical fiber 142 is boosted withthe superimposed resilient force of the spring 147 of the shape memoryalloy, and the optical fiber 142 is urged more effectively to thedirection of the laser diode chip 32.

[0801]FIG. 85 shows the optical connector formed of the tapered splitsleeve 1431, the ferule 142A holding the optical fiber 142, the coilspring 1432 applying an axial urging force to the ferule 142A, thecollar 1433, a plug housing 1434 accommodating therein the split sleeve1431, the coil spring 1432 and the collar 1433, and the adapter housingaccommodating the ferule 142B and the plug housing 147, wherein theoptical connector having such a construction is connected with anotheroptical connector by connecting the respective connector housings 143with each other by way of a connecting screw 1436.

[0802] In the construction of FIG. 65, it should be noted that the coilspring 1432 exerts an urging force at the normal temperature as a resultof the initial shape thereof. Thus, the coil spring 1432 is extended atthe normal temperature change, and the ferule 142A is urged in thedirection of the axis of the optical fiber 142. Thereby, two opticalfibers are connected effectively. In this example, too, it is possibleto use various shape memory alloys and shape memory plastics. In theexample noted above, the present invention was explained with referenceto the example that uses a shape-memory alloy for the spring. However,it is possible to use an ordinary resilient material such as copperbronze or urethane foam. By using such well-known art, it is possible toreduce the cost of the module.

[0803] [Fifteenth Embodiment]

[0804] Next, a further embodiment of the present invention will beexplained.

[0805]FIG. 86 shows an example of the construction of the opticaltransmission part that is provided with the optical telecommunicationsystem that uses the surface-emission laser diode of the presentinvention. In FIG. 86, those parts explained previously are designatedby the same reference numerals and the description thereof will beomitted.

[0806]FIG. 86 is referred to.

[0807] In the present embodiment, a connector substrate 151 holdingplural optical fibers 142 on the mount substrate 131 is provided suchthat each of the plural surface-emission laser diodes 32 forming a partof the array opposes with a corresponding one the plural optical fibers142. Further, plural photodetection devices 142P are provided on theconnector substrate 151 so as to monitor the optical power of thesurface-emission laser diodes 32, such that the photodetection devices142P correspond respectively to the plural optical fibers 142. Each ofthe photodetection device 142P is provided as it face the correspondingsurface-emission laser diode 32 for detecting the optical beam emittedtherefrom.

[0808] In FIG. 86, it can be seen that the laser diode chips 32, theoptical fibers 142 and the monitoring photodetectors 142P are arrangedto form a one-dimensional array. However, a similar effect is achievedalso when the photodetectors 142P are disposed in a two-dimensionalarray. In this drawing, each of the laser diodes is represented as anindependent chip 32. However, the laser diodes may be the one formed ona common chip 32.

[0809] In FIG. 86, illustration of the driver circuit of thesurface-emission laser diode 32 or the laser output control circuit forfeedback control of the laser output based on the signals acquired fromthe driver circuit or monitoring photodetection device is omitted forthe sake of simplicity.

[0810] In the example of FIG. 86, the connector substrate 151 holds theoptical fiber 142 and is mounted on the mount substrate 131 by using aconnector guide 151A.

[0811] Each of the optical fibers is held in a penetrating hole providedin the connector substrate surface 151 so that the end surface of theoptical fiber faces a corresponding laser diode element 32A in the laserdiode chip 32. Further, each laser diode elements 32A and thecorresponding optical fiber 142 are aligned optically. Further, there isprovided a monitoring photodetector 142P in the vicinity of the opticalfiber insertion hole of the connector substrate 151.

[0812] Here, the majority of the laser beam emitted from thesurface-emission laser diode is injected into the optical fiber 142aligned to the laser diode. On the other hand, the part of the laserbeam failed to enter the optical fiber 142 is detected by thephotodetection device 142P.

[0813]FIG. 87 shows the block diagram of the control system thatstabilizes the output of the surface-emission laser diode 32 by usingthe monitoring photodetector 142P shown in FIG. 86.

[0814] Referring to FIG. 87, the surface-emission laser diode 32 isdriven by a laser drive circuit 161 to which the data signal (electricalsignal) is supplied, and there is formed a laser bean as a result. Apart of the laser beam thus formed is detected as the leakage light, andthe output electrical signal of the photodetector 142P is supplied to afeedback control circuit 162. The feedback control circuit 162 in turncontrols the laser driver circuit 161 so as to compensate for the changeof the output electrical signal of the photodetection device 142.

[0815] It should be noted that no strong optical power is needed for thelaser beam detected by the monitoring photodetector 142P, as themonitoring photodetector merely detects the change of the laser outputfor feedback control. From the viewpoint of long range opticaltelecommunication, it is preferable that most of the optical power isinjected into the optical fiber, while the system of using the feebleleakage light at the incident end of the optical fiber 142 issufficiently practical.

[0816]FIG. 88 shows another embodiment of the present invention.

[0817] Referring to FIG. 88, the monitoring photodetector 142P is formedbetween the surface emission laser diode chip 32 and the connectorsubstrate 151 in the vicinity of the incident end of the optical fiber142 facing the laser diode chip 32. Similar to FIG. 86, the laser diodes32, the optical fibers 142 and the photodetection devices 142P arearranged in a one-dimensional array, while the present invention is notlimited to such a specific case but is applicable also to the case inwhich there is formed a two-dimensional array on the mount substrate131. Similarly to the explanation above, each of the laser diodes isrepresented as a chip, while it is also possible to form a number oflaser diode emission parts 32A on a single chip 32 monolithically as alaser diode element. In view of simplicity of illustration, the drivercircuit and the laser output control circuit are not illustrated.

[0818] In the example of FIG. 88, the monitoring photodetector 142P isprovided between the substrate 131 and the connector substrate 151 inthe form supported on a support member 142W, contrary to the case ofFIG. 86.

[0819]FIG. 89 shows a further embodiment of the present invention.

[0820] Referring to FIG. 89, the monitoring photodetectors 142P aredisposed on the mount substrate 131 together with the surface-emissionlaser diode chip 32 and detects a part of the optical beam reflected bythe connector substrate 131. In this embodiment, too, it is possible toform a number of laser diode elements 32A in a laser diode chip 32 andform the photodetectors 142P also on the chip 32 monolithically adjacentto the laser diode elements 142P.

[0821] According to the latter construction, the laser diode elements32A and the photodetectors 142P are formed simultaneously and with highprecision. Further, the cost of assembling is reduced. In FIG. 89, too,the illustration of the driver circuit of the laser diode 32 or theoutput control circuit is omitted for the sake of simplicity.

[0822] In the present embodiment, most of the laser beam emitted fromthe surface-emission laser diode 32 is injected into the optical fiber142 coupled optically to the laser diode 32. Thereby, the part of thelaser beam failed to enter the optical fiber 142 is reflected at thebottom surface of the connector substrate and enters the photodetectiondevice 142P. Thus, in the present embodiment, too, it is possible tocontrol the output of the laser diode 32 by using the leakage light atthe incident end of the optical fiber 142.

[0823] Next, another embodiment of the present invention will bedescribed with reference to FIG. 90.

[0824] Referring to FIG. 90, it can be seen that the relationshipbetween the laser diode 32 and the monitoring photodetector 142P is thesame as in the case of FIG. 89 except that there is provided a reflector151R at the bottom surface of the connector substrate 151 for improvingthe efficiency of reflection.

[0825] Such a reflector 151R can be formed by providing a metal film ofAl, Ag or Au on the bottom surface of the connector substrate 151 thatfaces the laser diode 32. In order to reflect the optical beam of longwavelength, it is preferable to use a metal film of Au.

[0826] In the present embodiment, too, it is possible to control thelaser output of the surface-emission laser diode by using a leakagelight which has failed to enter the optical fiber in the vicinity of theoptical fiber, similarly to the previous embodiments. As a result of theuse of reflector, the optical output control is achieved with improvedprecision with weaker power of the reflection light.

[0827] [Sixteenth Embodiment]

[0828] Next, a further embodiment will be described.

[0829]FIG. 91 shows an example of the optical telecommunication systemthat uses a long-wavelength laser diode of the present invention. Morespecifically, FIG. 91 shows the construction in which the optical fiber72 extending out from the module package 71, in which the laser diodechip 32 is accommodated, is connected to the optical fiber 73 of theoptical telecommunication system. In FIG. 91, those parts correspondingto the parts described previously are designated by the same referencenumerals and the description thereof will be omitted.

[0830] Referring to FIG. 74, the optical fiber 72 drawn out from themodule package 71 is connected to the optical fiber 73 used for opticaltelecommunication at the optical connector 171. When there exists air atthe connection part, there arises a reflection of the optical beam inview of the difference of refractive index of the air with respect tothe refractive index of the optical fiber 72 or 73. In order to suppresssuch a fluctuation of refractive index a the optical fiber connectionpart, there are proposals to inject a refractive index matcher in theform of a gel so as to suppress the fluctuation of the refractive indexor to achieve a complete attachment of the optical fibers (PC: physicalcontact). The present invention uses this PC technology for theconnection of the optical fibers.

[0831] As represented in FIG. 91, the optical fibers 72 and the opticalfibers 73 are held in the connector 171 respectively by a ferule 72F anda ferule 73F for preventing damaging of the optical fibers. The ferules72F and 73F are formed of a cylindrical body of a zirconia ceramic thathas a large mechanical strength. Thereby, the optical fibers areinserted into the central hollow part, and fixed therein by using anadhesive. Further, the ferules 72F and 73F are held in the opticalconnector 171 by a pair of split sleeves 172.

[0832]FIG. 92 shows the relationship between the optical fibers 72 and73, the ferules 72F and 73F and the split sleeve 172.

[0833] Referring to FIG. 92, the split sleeve 172 consists of a pair ofmembers formed of a copper bronze and having the shape of being splitfrom a hollow cylinder.

[0834] As represented in FIG. 92, a pair of split sleeve members 172holds the ferules 72F and 73F, and the ferules 72F and 73F are urgedwith each other by respective springs 173A and 173B with a predeterminedforce. Thereby, there is formed the desired PC connection. In thepresent embodiment, it is possible to construct a rigid and highlyreliable optical connection.

[0835] Generally, a laser diode is provided with a safety standard forpreventing damages to human body. In the case of the surface-emissionlaser diode of 1.1-1.7 μm, it is possible to satisfy the safety standardwhile operating the laser diode with a large output power as comparedwith the laser diode having the oscillation wavelength in the visiblewavelength band (0.4-0.7 μm).

[0836] [Seventeenth Embodiment]

[0837] Next, another feature of the present invention will be describedof the coupling between the laser diode and optical fiber or laser diodeand optical waveguide.

[0838] As represented in FIG. 93, the laser beam of the surface emissionlaser diode emitted perpendicularly to the substrate has a opticalintensity distribution generally close the Gauss distribution profile inthe plane perpendicular to the optical axis.

[0839] Assuming that the beam diameter is equal to the FWHM (full-widthhalf-height) value of the beam profile, it is possible to evaluate theemission angle θ of the optical beam from the beam diameter and thedistance between the laser diode and the detection surface.

[0840] As a result, it turned out that the emission angle θ of thesurface-emission laser diode of the present invention is generallysymmetric about the optical axis and takes a value of 5-10 degrees.

[0841] In contrast to this, conventional laser diode of theedge-emission type having the same oscillation wavelength of 1.1-1.7 μmhas a much larger emission angle θ and a very large aspect ratio.Typically, the optical emission angle θ⊥ has a value of about 35 degreesin the direction vertical to the plane of the substrate and the opticalemission angle θ// parallel to the place of the substrate takes thevalue of about 25 degrees. Thus, it has been necessary to use an opticalelement such as a microlens for realizing high efficiency opticalcoupling.

[0842] In contrast to this, in the case of the surface-emission laserdiode of the present invention, the angle of optical emission θ is verysmall as noted above, and the spreading of the optical beam is minimizedeven in the case there is a large distance between the laser diode andthe optical fiber or between the laser diode and the optical waveguide.Thereby, it becomes possible to eliminate the microlens, while theremoval of the microlens provides an beneficial effect in that thedistance between the laser diode and the optical fiber is reduced.Further, even in the case the distance between the laser diode and theoptical fiber or the laser diode and the optical waveguide is large, thespreading of the optical beam is small and an efficient optical couplingbecomes possible without providing an intervening lens.

[0843]FIG. 94 shows a schematic diagram with regard to spreading of thelaser bean in the laser diode 32 of the present invention and FIG. 95shows an example of calculation of beam spreading.

[0844] Referring to FIG. 94, the laser emission part 32A of the laserdiode 32 has an aperture size d [μm] and the laser beam is emitted fromthe laser emission part 32A with an emission angle θ. Further, there isprovided an edge surface of the optical beam 180 at the positionseparated from the laser diode 32 with an optical length l [μm]. Theoptical fiber 180 includes a core 181 having a diameter X [μm] and acladding 182 surrounding the core, and the optical fiber 180 is providedso as to align the laser diode optically such that the optical axis ofthe optical fiber 180 and the optical axis of the laser diode coincidewith each other. Here, it should be noted that the diameter d of theaperture represents the diameter itself when the diameter has a circularshape, while in the case the diameter has a square shape, the diameter drepresents the length of one edge. Further, when the opening has othershape such as a rectangle, the diameter d represents the diameter of acircle having an area of an ellipse inscribing the rectangular opening.

[0845] The reason that the optical path length between the laser diodeand the optical fiber edge or optical waveguide edge is to include thecase of using a mirror for deflecting the optical beam to be describedlater with reference to FIG. 97. When there is no such a deflection ofthe optical path and the optical beam propagates straight, the opticalpath length from the laser diode to the optical fiber edge or theoptical waveguide edge becomes identical with the distance from thelaser diode to the optical fiber edge or the optical waveguide edge. Inthe case of an optical waveguide, it is general that the corecross-section is not a circle but a square or rectangle. Thus, in thecase of a square core, the edge size is set as X [μm], while in the caseof a rectangular core, the size of the shorter edge is defined as X[μm].

[0846] In FIG. 94, it can be seen that the beam size of the optical beamemitted from the laser diode perpendicularly to the epitaxial layerstherein, the beam diameter of the laser beam increases with the opticalpath length L and the optical emission angle θ until it reaches the edgesurface of the optical fiber or optical waveguide. Thereby, the beamsize at the optical fiber edge or the optical waveguide edge is given as

d+21 tan(θ/2)

[0847] Thus, when this beam size falls in the core diameter X of theoptical fiber or the optical waveguide, excellent optical coupling isachieved.

[0848] Based on the above equation, FIG. 95 shows the relationshipbetween the optical path length l and the beam diameter for the case ofthe aperture diameter of 5 μm and the optical emission angle of 10degrees, in comparison with the conventional case of an edge-emissiontype laser diode having an optical emission angle of 35 degrees.

[0849] Referring to FIG. 95, it can be seen that the spreading of thebeam diameter is substantially small as compared with the conventionalcase and that it is possible to coincide the core diameter X and thebeam diameter in the case a commonly used multimode optical fiber havinga core diameter of 50 μm (cladding diameter of 125 μm) is used for theoptical fiber, provided that the optical path length l is set to 260 μm.

[0850] As long as the optical path length l is smaller than theforegoing length in which the core diameter X coincide with the beamdiameter, it is possible to achieve an excellent optical coupling withlittle optical loss. From the result of FIG. 95, it is concluded thatthe optical alignment between the laser diode and the optical fiber oroptical waveguide is not critical in the axial direction in the case ofthe laser diode of the present invention, and a very coarse alignment issufficient for achieving the necessary optical coupling. In the case ofa plastic optical fiber having a core diameter of 100 μm is used, theoptical path length l in which the laser beam diameter coincides withthe core diameter increases further to 550 μm. Thereby, it is possibleto provide the optical fiber with a separation from the laser diodepackage.

[0851]FIG. 96 shows an example of coupling an optical fiber of anoptical telecommunication system with the surface-emission laser diodeof the present invention. In FIG. 96, those parts corresponding to theparts described previously are designated by the same reference numeralsand the description thereof will be omitted.

[0852] Referring to FIG. 96, the telecommunication system includes themount substrate 131 carrying thereon the surface-emission laser diodechip 32 in which the laser diodes 32A are formed as a two-dimensionalarray, and the mount substrate 131 further carriers a driver circuit ofthe laser diode not illustrated. Further, there are provided atwo-dimensional array of photodetectors (not shown) as well as thedriver circuit not illustrated cooperating with the photodetectors,wherein an optical fiber array including optical fibers 191 is providedon the laser diode chip 32 such that each of the optical fibers 191 facea corresponding laser diode 32A. It is also possible to use an opticalwaveguide array in place of the optical fiber array in the constructionof FIG. 96. Further, the optical telecommunication system of FIG. 96 maybe a bi-directional optical telecommunication system. In this case, theoptical transmission part and photodetection part are provided at bothends of the optical fibers.

[0853] Referring to FIG. 96, the mount substrate 131 is formed of athermally conductive Si substrate and the laser array chip 32 is mountedon the mount substrate 131. The laser array chip 32 may contain thelaser diode of FIG. 1 as a laser diode element constituting the array.In the illustrated example, the laser diodes having an oscillationwavelength of 1.3 μm is used, wherein each of the laser diodes 32A hasan aperture having the diameter of 10 μm for optical output. Twelvelaser diodes 32A are formed on the mount substrate 131 with a pitch of200 μm.

[0854] On the mount substrate 131, there is provided a hole array member191 having a number of penetrating holes with a pitch of 200 μm incorrespondence to the pitch of the laser diodes 32A on the chip 32,wherein each of the holes has a diameter of 125 μm in correspondence tothe diameter of the multimode optical fibers. The hole array member 191may be formed of AlN and carries a guide 191X in correspondence to amarker 32X provided on the laser diode chip 32. Thereby, the hole arraymember 191 is mounted on the laser diode chip 32 such that the guide191X coincides with the marker 32X. The hole array member 191 has anexcellent thermal conductivity of about 300 Wm⁻¹K⁻¹ and is used as aneffective heat radiation part when it is provide close to the laserdiode chip 32. Thereby, a stable optical telecommunication system isrealized by dissipating the heat associated with the laser oscillationefficiently.

[0855] In each of the penetrating holes of the hole array member 191, amultimode optical fiber 192 is inserted and the optical fiber is fixedtherein by an adhesive in the state that the optical fiber engages thelaser diode chip 32. With such a construction, it is possible to achievean exact alignment between the laser diode element 32A and the opticalfiber 1691 in the direction perpendicular to the optical axis direction.

[0856] As noted above, the optical fiber 191 is merely contacted withthe chip 32 and the accuracy of optical alignment in the axial directionis poor, There cab be a case in which a space of 0-20 μm may be formedbetween the laser diode and the optical fiber. However, because of thevery small emission angle θ of the laser diode of 8 degrees in thepresent example, and in view of the large diameter d of 10 μm for thelaser diode 32A, the optical beam diameter is only 17 μm at the distanceof 50 μm from the surface of the laser diode chip 32. Thus, the beamdiameter is still much smaller than the core diameter of 50 μm for themultimode optical fiber 192, and excellent optical coupling ismaintained. Further, there is no need of providing a coupling lensbetween the laser diode 32A and the optical fiber 191.

[0857] In the experiment conducted on the construction of FIG. 96 inwhich the optical fiber 191 is pulled out gradually after beingcontacted with the laser diode chip 32 by using a micrometer, it wasconfirmed that the laser diameter becomes 66 μm when the distancebetween the laser diode and the optical fiber is 66 μm. In this case,the optical beam diameter is larger than the core diameter of themultimode optical fiber 191 and the foregoing relationship is failed.Thus, no sufficient optical coupling would be achieved in this case.

[0858] Table 7 below represents the experiments conducted by theinventor. TABLE 7 d + 21 tan(θ/2) [μm] X (μm) remarks 10 50 ◯ 20 50 ◯ 3050 ◯ 40 50 ◯ 50 50 ◯ 60 50 Δ 70 50 X 80 50 X 10 62.5 ◯ 20 62.5 ◯ 30 62.5◯ 40 62.5 ◯ 50 62.5 ◯ 60 62.5 ◯ 70 62.5 X 80 62.5 X

[0859] From the result of above, it can be seen that a practical opticalcoupling cannot be achieved unless the core diameter X is larger thanthe term given as d+21 tan(θ/2), where θ is the emission angle of theoptical beam and l represents the optical path length from the laserdiode to the optical fiber, d represents the diameter of the laser diodeopening.

[0860] In the explanation above, it should be noted that the number ofthe optical fibers is not limited to 12 but there may be only oneoptical fiber or there may be 4, 8, 16 optical fibers. Further, thepresent invention is not limited to a multimode fiber 191 b but a singlemode optical fiber may also be used for long distance transmission.Further, it is possible to use a plastic optical fiber (POF), which isadvantageous to construct a short distance, low const opticaltelecommunication system. In such a system, it is preferable to providean anti-reflection coating on the edge surface of the optical fiber. Inthe present embodiment, it is also possible to use Si or C or aluminaceramics for the hole array member 191. As the hole array member 191functions also as a heat radiation part, it is preferable to use amaterial having a high thermal conductivity.

[0861] [Eighteenth Embodiment]

[0862] Next, another feature of the present invention will be describedof the coupling between the laser diode and optical fiber or laser diodeand optical waveguide.

[0863] The construction of the present invention is similar to that ofFIG. 96 except that the lser diode chip 32 and the optical fiber arecontacted with each other and the distance between the laser diode chip32 and the optical fiber 191 or optical waveguide is set to almost zero.Here, the representation “contact” also includes a separation of 0-10 μmby taking into consideration of the error at the time of the assemblingprocess.

[0864] In the present embodiment, a Si substrate having good thermalconductivity is used for the mount substrate 131 and the laser arraychip 32 including an array of surface-emission laser diodes 32A ismounted on the mount substrate 131. The laser diode 32A used in thepresent embodiment has a laser oscillation length of 1.3 μm.

[0865] In the present embodiment, the diameter of the laser diode is setto 10 μm and 12 laser diodes 32A are formed with a pitch of 200 μm.

[0866] Next, the multimode optical fibers 192 are inserted into theholes in the hole array member 191 having the penetrating holes of 125μm diameter with the corresponding pitch of 200 μm. Thereby, the tipends of the optical fibers 192 are projected slightly from the holearray member 191 such that the tip end of the optical fiber makes anengagement with the laser array chip 32.

[0867] Next, the tip end part of the optical fiber is polished, and thetip end of the optical fibers is aligned at the side of the hole arraymember 191 that makes a contact with the laser diode chip 32. Afterthis, the marker X and the guide 191X are matched and the opticalalignment is achieved between each of the laser diodes 32A and thecorresponding optical fiber 192. The laser array chip 32 and the holearray member 191 are then fixed in this state, and the laser diode chip32A and the optical fiber 192 are coupled with each other.

[0868] According to the present embodiment, an optical coupling betterthan the previous embodiment is achieved. Thus, in the presentembodiment, there occurs little spreading of the optical beam and thelaser beam diameter is more or less coincident with the diameter d ofthe aperture forming the laser diode 32A. Thus, the diameter of thelaser beam is sufficiently smaller than the core diameter of the opticalfiber 191 and the alignment margin in the axial direction is increasedfurther. Thereby, it becomes possible to construct an optical systemusing a 1.3 μm surface emission laser diode with low cost.

[0869] [Nineteenth Embodiment]

[0870]FIGS. 97A and 97B are diagrams showing the coupling of the laserdiode and an optical wave guide according to another embodiment of thepresent invention, wherein FIG. 97A shows an oblique view while FIG. 97Bshows a cross-sectional view.

[0871] In the present embodiment, the laser beam emitted from a laserdiode 32A is deflected by a mirror 301. In FIGS. 80A and 80B, thoseparts corresponding to the parts described previously are designated bythe same reference numerals and the description thereof will be omitted.

[0872] Referring to FIGS. 97A and 97B, a Si substrate having anexcellent thermal conductivity is used for the mount substrate 131, andthe laser array chip 32 in which the laser diodes 32A are arranged inthe form of an array is mounted on the mount substrate 131. In thepresent embodiment, each of the laser diodes 32A has an aperture with adiameter of 15 μm, and four laser diodes 32A are formed.

[0873] In this embodiment, another mount substrate 131A is provided inwhich a mirror 301 is formed by mounting a reflecting member or forminga reflecting layer monolithically. Further, an optical waveguide 302 isformed on the mount substrate 131A. For example, the mirror 301 may beformed by applying an anisotropic etching to the Si substrate by using aKOH etchant and by providing an Ag film on the crystal surface formed asa result of the anisotropic etching.

[0874] In the present embodiment, an optical waveguide 302 is formed onthe mount substrate 131A. The optical waveguide 302 is formed bydepositing a lower cladding layer 302A, followed by depositing apolymethacrylate (PMMA), followed further with a patterning process toform a core pattern 302B. Typically, the core pattern 302B has a crosssectional form of 50×50 μM. Other than PMMA, the optical waveguide layermay be formed also by polyimide or epoxy resin or polyurethane orpolyethylene. Further, it is also possible to form an inorganic filmsuch as silicon oxide film. Further, these organic films may be formedby a spin-coating process of dip-coating process in combination with apatterning process. Further, it is possible to form the opticalwaveguide by a resin molding process or molding process.

[0875] In the present embodiment, the mount substrate 131 and the mountsubstrate 131A carrying the optical waveguide 302 are fixed such thatthe optical axis of the laser diode 32A and the optical axis of theoptical waveguide 302 become coincident.

[0876] In the experiments conducted by the inventor, a laser diodehaving an aperture of 15 μm diameter and the optical emission angle θ of10 degrees is used, and the optical path length l between the laserdiode 32A and the optical waveguide 302 is changed by setting theposition of the end surface of the optical waveguide 302 to 50 μm, 100μm and 250 μm. As a result, it was confirmed that an excellent result isobtained in the case the optical path length is 50 μm and 100 μm, whileno satisfactory optical coupling was achieved in the case the opticalpath length is set to 250 μm.

[0877] Anyway, the present embodiment can eliminate the lens and only acoarse optical alignment is sufficient for the axial direction. Thus, byusing a surface-emission laser diode of 1.2 μm, it becomes possible toconstruct an optical alignment having a tolerance with regard to theoptical alignment in the axial direction or the direction of the opticalaxis.

[0878] In the present embodiment, the mirror 301 is formed separately tothe optical waveguide 302. On the other hand, it is possible to form a45 degree surface at the end of the optical waveguide 302 by a dicingblade, and the like, and by depositing a metal film such as Ag on suchan oblique surface. Thereby, a mirror integral with the opticalwaveguide 302 is formed.

[0879] In the present embodiment, the diameter of the optical waveguideis set to 50 μm in correspondence to core diameter of the multimodeoptical fiber. In the case a single mode optical fiber is used, thediameter of the optical waveguide has to be set to about 10 μm. In thiscase, too, the same fundamental equation holds. Further, it should benoted that the cross sectional form of the core pattern 302 in theoptical waveguide 302 is not limited to the square pattern but arectangular pattern may also be used. Further, it is possible totransmit plural optical signals in a single optical waveguide as in thecase of an optical sheet.

[0880] [Twentieth Embodiments]

[0881]FIG. 98 shows the construction of coupling the laser diode chip 32with the optical waveguide 302 according to another embodiment of thepresent invention.

[0882] Referring to FIG. 98, a Si substrate having excellent thermalconductivity is used for the mount substrate 310 and the laser diodechip 32 carrying the laser diodes 32A of the structure of FIG. 1 ismounted on the substrate 310. In the present embodiment, too, asurface-emission laser diode having an oscillation wavelength of 1.3 μmis used for the laser diode 32A. The laser diode used in the presetembodiment has the aperture diameter d of 7 μm and the optical emissionangle θ of 8 degrees. In the present embodiment, the laser diode 32A isused as a single device, not in the form of array.

[0883] The mount substrate 310 carrying thereon the laser diode chip 32is fixed on a package body 311 with positioning. Further, the opticalaxis of the laser diode 32 is aligned with the optical axis of thesingle mode optical fiber 313 held on an optical fiber guide 312, byaligning the optical fiber guide 312 and the package 311 in advance byusing a guide 311A on the package body 311 and a guide pin 312A on theoptical guide 312.

[0884] The single mode optical fiber 313 includes a core 313A having adiameter of 10 μm and a clad having a diameter of 125 μm and surroundingthe core 313A, wherein the diameter of the optical fiber guide 312 isset so as to coincide with the outer diameter of the cladding of theoptical fiber 313.

[0885] In the construction of the present embodiment, an excellentoptical coupling is achieved by contacting the single mode optical fiber313 to the laser diode 32. Further, the present invention enables abroadband optical signal transmission by using the single mode opticalfiber 313 over a long distance. Thus, according to the presentembodiment, it became possible to construct an optical telecommunicationsystem having excellent optical coupling by using the surface-emissionlaser diode of the 1.3 μm band.

[0886] [Twenty-First Embodiment]

[0887] Next, another embodiment of the present invention will bedescribed.

[0888]FIG. 99 shows an example of the optical telecommunication systemthat uses a long-wavelength laser diode according to another embodimentof the present invention.

[0889] Referring to FIG. 99, the optical transmission system includes asurface-emission laser diode 32 having the emission part 32A and anoptical fiber 352, wherein the laser diode 351 and the optical fiber 352are coupled directly at the laser emission part 32A. Of course, it ispossible to achieve the optical coupling by using a lens.

[0890] Assuming that the laser emission part 32A of the surface-missionlaser diode 32 is d and the core diameter of the optical fiber 312 as F,it can be seen that the laser beam from the laser diode 32 diverges asrepresented in FIG. 99 by a broken line 3515. In FIG. 99, it should benoted that d represents the diameter of the circle inscribing thepolygonal laser emission part 32A. In this case, the emission angle ofthe laser beam is much smaller than the case of a conventionaledge-emission laser diode. Thus, in the case the laser diode 32 isprovided near the optical fiber, it should be noted that a high opticalcoupling can be achieved when the optical axis of the laser diode isperpendicular to the optical fiber end surface and is coincident withthe optical axis of the laser diode, and the aperture diameter of thelaser diode is equal to or smaller than the core diameter.

[0891] As noted before, the surface-emission laser diode used in thepresent embodiment operates in the wavelength band of 1.1-1.7 μm, and along distance transmission becomes possible by using the wavelength bandof 1.3-1.55 μm due to the small optical loss of the quartz optical fiberin this wavelength band.

[0892] Assuming that the laser emission part 32A of the laser diode 32has the diameter of d (in the case the laser emission part 32A has apolygonal shape, the diameter d is the diameter of a circle inscribingthe polygonal laser emission part 32A, it is possible to increase theoptical coupling efficiency by setting the foregoing parameters F and dso as to satisfy the relationship of

F/d≦2  (2)

[0893] In the embodiment of FIG. 99, it should be noted that the laseremission part 32A and the optical fiber 352 are coupled directly and nolens is interposed.

[0894] In the conventional optical system that uses an edge emissiontype laser diode of the 1.3 μm band, there is been a poor opticalcoupling efficiency between the laser diode and the optical fiber, andit has been difficult to couple these directly. Further, the return beamincident to the laser dude from the optical beam in the backwarddirection has caused a change oscillation state of the laser diode.

[0895] In the case of the present invention, the laser diode is asurface-emission laser diode and operates in the long-wavelength band.Thus, the laser diode has a generally circular beam shape characterizedby excellent aspect ratio close to 1. Thereby, the efficiency of thelaser diode is improved substantially as compared with the case of theedge-emission type laser diode.

[0896] Further, the laser diode of the present invention has thereflector of very high reflectance and the effect of the return beam issuccessfully suppressed. Thereby, it becomes possible to couple thelaser diode and optical fiber directly, and the optical isolator used inthe conventional optical telecommunication system can be eliminated.

[0897] Designating the diameter of the laser emission part 32A as d andthe optical fiber core diameter as F, it is possible to increase theoptical coupling efficiency by setting the parameters d and F so as tosatisfy the relationship

0.5≦F/d≦2  (3)

[0898] Hereinafter, the reason of this will be explained with referenceto Table 8, which summarizes the result of the study of the inventor ofthe present invention on the optical coupling loss. TABLE 8 F/d couplingloss (dB) performance 0.4 >5 X 0.5 3-5 ◯ 0.65 1 ◯ 2.0 3-5 ◯ 2.2 >5 X

[0899] Referring to Table 8, it can be seen that the efficiency ofoptical coupling is decreased when the diameter d of the beam emissionpart 32A has exceeded the core diameter F. On the other hand, it can beseen also that as long as there is a relationship d≦2F (in other words0.5≦F/d), the optical coupling loss can be suppressed within 3-5 dB. Inthe case the diameter d of the beam emission part 32A is smaller thanthe core diameter, a highly efficient optical coupling is achieved inthe case the emission angle of the laser diode, determined by thediameter of the circle inscribing the beam emission part 32A and thewavelength, is smaller than the numerical aperture NA for allowing asingle mode optical coupling to a single mode optical fiber.

[0900] For example, in the case the core diameter is set to 10 μm andthe core refractive index is 1.4469 and the clad refractive index if1.4435, the numerical aperture NA that enables single mode opticalcoupling becomes 0.0995 at the wavelength of 1.3 μm. The diameter d ofthe beam emission part 32A corresponding to this is about 6.5 μm.

[0901] On the other hand, even in the case the diameter of the beamemission part 32A is less than 6.5 μm, it is possible to suppress theoptical coupling loss to the level of 3-5 dB when the diameter of thebeam emission part 32A is about 5 μm. From this, it is concluded that itis preferable to choose the parameters F and d to satisfy therelationship

0.5≦F/d≦2.

[0902] While the foregoing arguments have been for the case of using asingle mode optical fiber, the foregoing condition is applicable also tothe case of using a multimode optical fiber coupling a single modeoptical fiber via a tapered optical waveguide.

[0903] Next, another example of the optical telecommunication system ofthe present invention will be described with reference to FIG. 100.

[0904] Referring to FIG. 100, the present embodiment uses a couplinglens 353 between the surface-emission laser diode 32 and the single-modeoptical fiber. The lens 353 may be formed of a single lens or a lenssystem in which a plurality of lenses are combined. In the case ofconstructing the lens 353 by a single lens, it is preferable to disposethe lens 353 close to the beam emission part 32A of the laser diode chip32.

[0905] In the construction of FIG. 100, it becomes possible to suppressthe optical coupling loss effectively even in the case the diameter d ofthe beam emission part 32A is set larger than the core diameter of theoptical fiber 353, by appropriately choosing the lens power of thecoupling lens 353.

[0906] In the case the core diameter is 10 μm and the diameter d of theemission part 32A is 20 μm, for example, the beam size is reduced by ½by the lens 353. Thus, designating the focal distance of the lens 353 asf, the laser wavelength as A, the radius of the beam emission part 32Aas ω₀ (=10 μm) and the refractive index as n, the focal distance f isobtained by the relationship$\frac{\frac{\lambda \cdot f}{\pi \quad \omega_{0}^{2}n}}{\sqrt{1 + \left\lbrack {1 + \frac{\lambda \cdot f}{\pi \quad \omega_{0}^{2}n}} \right\rbrack^{2}}} = \frac{5}{10}$

[0907] In this case, the focal distance f becomes about 140 μm.

[0908] In the case of using a coupling lens, it is possible to achievean efficient optical coupling when there holds a relationship betweenthe parameters d and f noted before as

d≦f, or

F/d≦1.  (4)

[0909] Further reference should be made to Table 9 below. TABLE 9 F/dcoupling loss (dB) performance 1.2 >5 X 1.0 3-5 ◯ 0.8 1 ◯

[0910] In the embodiment of FIG. 100, the coupling lens 353 was providedby a single lens. On the other hand, it is possible to construct thelens 353 by a plurality of lenses. For example, as represented in FIG.101, the lens 353 may be formed of two lenses 353A and 353B.

[0911] Referring to FIG. 101, the diverging beam from the beam emissionpart 32A is focused by the first lens 353A to the second lens 353B, andthere is formed a beam waist at the plane of the second lens 353B. Thelens 353B functions as the lens 353 of FIG. 100. According to thepresent embodiment, it becomes possible to maintain a high couplingefficiency in the case a single lens cannot be disposed near the beamemission part of the laser diode or in the case the divergence of thebeam is excessive and the optical coupling efficiency is decreased, byconstructing the lens 353 by plural lenses.

[0912] In the description heretofore, it was assumed that a single modeoptical fiber is used for the optical fiber 352. On the other hand asimilar effect is achieved also in the case of using a multimode opticalfiber or a tapered optical waveguide, as long as the relationship ofEquation (4) is satisfied.

[0913] In the construction that uses a conventional edge-emission laserdiode, there is a possibility that the laser oscillation is influencedby the return beam from the coupling lens, and it has been necessary toprovide an optical isolator. In the case of the surface-emission laserdiode, there is provided a reflector structure of high reflectance andthe effect of the return beam is successfully eliminated. Thus, itbecomes possible to eliminate the optical isolator.

[0914] Next, another embodiment constructed by a surface-emission laserdiode and the optical fiber will be explained with reference to FIG.102.

[0915] Referring to FIG. 102, the laser diode chip 32 includes aplurality of beam emission parts 32A arranged in the form of array. Ofcourse, it is possible to dispose the laser diode chips 32 themselves inthe form of array. Further, a plurality of laser diode chips may bearranged in the form of array.

[0916] In FIG. 102, the optical fiber is coupled directly to thecorresponding optical emission part 32A, and thus, it is possible tosatisfy the Equation (3) explained before with reference to FIG. 99.Thereby, it becomes possible to construct an optical telecommunicationsystem of large capacity.

[0917]FIG. 103 is an embodiment in which a coupling lens is providedfurther to the construction of FIG. 102. In the construction of FIG.102, too, the relationship between the beam emission part 32A, the lensarray and the optical fiber 352 is the same as in the case of FIG. 102.Thus, by satisfying the Equation (4) by choosing the parameters d and F,an efficient optical coupling becomes possible. As the beam emissionpart 32A is arranged in the form of array, an optical telecommunicationsystem of large capacity is realized. In the present embodiment, it ispossible to combine a plurality of lenses in each of the coupling lens355.

[0918] [Twenty-Second Embodiment]

[0919] Next, another embodiment of the present invention will bedescribed.

[0920]FIGS. 104A and 104B show an example of the opticaltelecommunication system that uses a long-wavelength laser diode of thepresent invention as an optical source and shows the state in which thelaser diode chip 32 carrying the beam emission part 32A is connectedwith a laser driver IC 32D including a CMOS circuit, wherein FIG. 104Ashows a side view while FIG. 104B shows a plan view.

[0921] Referring to FIGS. 104Aa and 104N, the laser diode chip 32 andthe laser driver IC 32D are mounted on a conductive submount 401 by aconductive adhesive. Further, the laser diode chip and the laser driverIC 32D are connected by a high-frequency transmission line 402(microstrip line in the present embodiment), and each chip 32 isconnected to this microstrip line 402 by way of a bonding wire 403.

[0922] In the case of the surface-emission laser diode for opticaltelecommunication, it is necessary to carry out a very fast modulationin the order of several hundred MHz to several GHz. Thus, it ispreferable to decrease the distance between the laser diode chip 32 andthe laser driver IC 32D. On the other hand, there can be a case in whichsuch a construction is not possible because of the layout of the opticalsystem. When an ordinary wiring is used in such a case for connectingthe laser diode chip 32 and the laser driver IC 32D, however, therearise s problem of electromagnetic emission from such wiring. In thepresent embodiment, the laser diode chip 32 and the laser driver IC 32Dare connected such that there occurs no such electromagnetic radiation.

[0923] By using such a construction, no electromagnetic emission occursfrom the wiring part and the need of providing special shielding meansis eliminated. Thereby, the cost of the optical telecommunication systemis reduced.

[0924] In the present embodiment, a microstrip line was used for thehigh-frequency transmission line 402. However, any of unbalancedtransmission line such as coplanar transmission line or triplate linemay be used. Further, the connection between the laser diode chip 32 andthe laser driver IC 32D is not limited to the bonding wired butflip-chip wiring or TAB bonding or microbump bondin may be used.

[0925] [Twenty-Third Embodiment]

[0926] In the embodiment of FIG. 52, a system that uses asurface-emission laser diode of long-wavelength band of 1.1-1.7 μm hasbeen explained. Conventionally, an optical telecommunication using thewavelength band of 0.85 μm band is studied. However, in this wavelengthband, the optical transmission loss is large in the optical fiber andpractical use of such a system was not successful. Further, there hasbeen no stable laser diode device in the long wavelength band in whichthe optical transmission loss is small. In the present invention, on theother hand, it became possible to construct a surface-emission laserdiode operable in the wavelength band of 1.1-1.7 μm by improving thesemiconductor distributed Bragg reflectors 12 and 18 and by providingthe non-optical recombination elimination layers 13 and 17, such thatthe laser diode operates stably at low power. Thereby, a practicaloptical telecommunication system has become the matter of reality

[0927] In the example of FIG. 52, the optical telecommunication systemis formed of the long-wavelength surface-emission laser diode chip, thefirst optical fiber FG1 injected with the laser beam emitted from thebeam emission part 32A in the chip 32 and acting as an opticaltransmission path, a second optical fiber FG2 injected with the laserbeam transmitted through the first optical fiber FG1 and functioning asan optical transmission path, a third optical fiber FG3 injected withthe laser beam transmitted through the optical fiber FG2 and functioningas an optical waveguide, and a photodetector chip 34 including aphotodetection part 34A coupled optically to the optical fiber FG3 fordetecting the laser beam transmitted through he optical fiber FG3.

[0928] Further, the optical connection module MG1 is provided betweenthe laser diode chip 32 and the first optical fiber FG1, while theoptical connection module MG2 is provided between the optical fiber EG1and FG2. Similarly, the optical connection module MG3 and FG4 areprovided between the optical fibers FG2 and FG3 and between the opticalfiber FG3 and the photodetector 34.

[0929] In the example of FIG. 52, the first optical fiber FG1 is coupleddirectly to the beam emission part 32A of the laser diode chip 34without intervening an optical system.

[0930] With reference to FIG. 94, a description was made on therelationship that is required between the laser diode of the presentinvention and the optical fiber.

[0931] In the long-wavelength surface-emission laser diode of thepresent invention, the optical emission angle of the laser beam is about10 degrees, and the laser diode is use din combination with an opticalfiber which may have a core diameter of 50 μm (clad diameter of 125 μm).

[0932] In such a combination of the laser diode and the optical fiber,the beam emission part 32A may have a size of 0.005 mm×0.005 mm−0.002mm×0.002 mm. In terms of area S, these are represented as 0.000025mm²−0.0001 mm². In this case, it is possible to achieve excellentoptical coupling without the need of providing an intervening opticalsystem. Further, the same result holds also in the case an opticalwaveguide is used in place of an optical fiber.

[0933] In the present invention, the beam emission part 32 is tuned tothe wavelength of 1.1-1.7 μm, while the foregoing wavelength band isrelated to the operational voltage of the laser diode.

[0934] Thus, in the present embodiment, a detailed analysis wasconducted on the operational voltage of laser diode 32 suitable forrealizing continues illation, not just a pulse oscillation.

[0935] Table 10 shows the experimental results of present embodiments.TABLE 10 emission size area S No. (mm × mm) (mm2) voltage V V/s remarks1 0.005 × 0.005 0.000025 0.2  8000 X 2 0.005 × 0.005 0.000025 0.3 12000X 3 0.005 × 0.005 0.000025 0.375 15000 ◯ 4 0.005 × 0.005 0.000025 0.416000 ◯ 5 0.005 × 0.005 0.000025 0.5 20000 ◯ 6 0.005 × 0.005 0.0000250.6 24000 ◯ 7 0.005 × 0.005 0.000025 0.75 30000 ◯ 8 0.005 × 0.0050.000025 0.9 36000 X 9 0.005 × 0.005 0.000025 1.2 48000 X 10 0.01 × 0.010.0001 1.2 12000 X 11 0.01 × 0.01 0.0001 1.3 13000 X 12 0.01 × 0.010.0001 1.5 15000 ◯ 13 0.01 × 0.01 0.0001 1.7 17000 ◯ 14 0.01 × 0.010.0001 1.9 19000 ◯ 15 0.01 × 0.01 0.0001 2.1 21000 ◯ 16 0.01 × 0.010.0001 2.3 23000 ◯ 17 0.01 × 0.01 0.0001 2.5 25000 ◯ 18 0.01 × 0.010.0001 3 30000 ◯ 19 0.01 × 0.01 0.0001 4 40000 X 20 0.01 × 0.01 0.0005 550000 X 21 0.02 × 0.02 0.0004 2  5000 X 22 0.02 × 0.02 0.0004 4 10000 X23 0.02 × 0.02 0.0004 6 15000 ◯ 24 0.02 × 0.02 0.00041 8 20000 ◯ 25 0.02× 0.02 0.0004 10 25000 ◯ 26 0.02 × 0.02 0.0000 12 30000 ◯ 27 0.02 × 0.020.00004 15 37500 X 28 0.02 × 0.02 0.00004 20 50000 X

[0936] From the results above, it can be seen that, by choosing theoperational voltage S such that the ratio of the operational voltage Vto the area S of the beam emission part 32A of the laser diode (V/S)falls in the range of 15000-3000, it can be possible to drive the laserdiode without causing a damage in the laser diode and the laser diodecan be operated continuously. By choosing the drive conduction as such,it is possible to realize a stable optical transmission system havingalong lifetime.

[0937] [Twenty-Fourth Embodiment]

[0938] Next, a further embodiment of the present invention will beexplained.

[0939]FIG. 105 shows the construction of an optical telecommunicationsystem using the long-wavelength laser diode according to the presentinvention, wherein those parts corresponding to the parts describedpreviously are designated by the same reference numerals and thedescription thereof will be omitted.

[0940] Referring to FIG. 105, the optical telecommunication system ofthe present invention has a construction to divide the optical output ofthe laser diode 32A provided on a mount substrate 301 into an opticalbeam for optical telecommunication and a monitoring beam by using themirror 301 provided on the mount substrate 301A, wherein the monitoringbeam thus divided is detected by the monitoring photodetector 142Pprovided on the mount substrate 301A. The monitoring photodetector 142Pcorresponds to the monitoring photodetector 142P explained previouslywith reference to FIG. 86. In FIG. 105, as well as in FIGS. 107 and 108to be described laser, there are formed a laser diode array by the laserdiodes arranged in the direction perpendicular to the plane of thedrawing. In correspondence to this, there are provided a plurality ofoptical waveguides.

[0941] The optical telecommunication system of the present embodiment isformed of an optical transmission part including the surface-emissionlaser diode 32 and the driver circuit thereof, the photodetection partincluding the planar photodetection unit and a drive circuit thereof,and an optical fiber or optical waveguide providing an opticaltransmission path extending between the optical transmission part andthe optical reception part. Although the driver circuit of the laserdiode 32 or photodetector 34 is not shown, these can be formed on thesubstrate of the respective devices. Alternatively, these can be formedintegrally with the laser diode device in the device fabricationprocess. By providing such an optical transmission part and opticalreception part at both ends of the optical transmission path, it becomespossible to realize a bi-directional optical telecommunication system.

[0942] As represented in FIG. 105, the laser beam emitted from a surfaceof the long-wavelength laser diode is divided by the mirror 301 and oneof the optical beams thus divided is then directed to the opticalwaveguide 302 provided with an optical alignment. This optical waveguide302 may be an optical fiber.

[0943] On the other hand, the other optical beam divided by the mirror301 is directed to the monitoring photodetection device 34 provided onthe mount substrate 301A. Thus, the mirror 301 is used to divide out thelaser beam to be supplied to the monitoring photodetector 34 and it ispreferable that such a mentoring optical beam is as weak as possiblewithin the range that a proper control of the laser diode is possible byusing such a monitoring device. In view of the power consumption of theoptical telecommunication system, it is preferable to provide as muchenergy as possible to the optical waveguide 302 as an optical signal.

[0944] Thus, in the present invention, the thickness of the metal filmof Au or Ag or Al used for the mirror 301 such that the transmittance ofthe mirror 301 is controlled for the wavelength of 1.1-1.7 μm. Further,it is also possible to provide openings of various shaped in the form ofgrooves, circles, squares, and the like, so as to control thetransmittance of the mirror 301. Thereby, in order to avoid the effectof unwanted optical interference, it is preferable to change the pitchor size or location of the openings at random. Further, it is possiblecontrol the transmittance of the mirror 301 by using a dielectricmultilayer mirror or semiconductor layered mirror.

[0945] As compared with the conventional laser diode of edge-emissiontype, the surface-emission laser diode used in the opticaltelecommunication system of the present invention has an advantageousfeature of small output dependence on temperature and little degradationwith time. While the laser diode of the present invention has suchadvantageous features, it is nevertheless preferable to control theoutput thereof by way of feedback control.

[0946] In the case of a conventional edge-emission laser diode, such amonitoring of the optical power of the laser diode was made easily bymonitoring the laser beam emitted from the rear edge surface of thelaser diode. In the case of the surface-emission laser diode, the laseroutput is obtained only at one side of the laser diode. Thus, theconstruction of monitoring the laser beam emitted in the backwarddirection cannot be used in the present invention. Further, the laserdiode ahs a relatively narrow angle of optical emission of about 10degrees. Thus, while there is a possibility of provide the laser diodeclose to the optical fiber or optical waveguide, there is no room at allfor inserting a monitoring photodetector between the laser diode and theoptical fiber.

[0947] On the other hand, the present invention provides an effective ansimple means of obtaining a monitoring laser diode by providing themirror 301 as noted above. Thereby, the output of the laser diode ismonitored with out increasing the optical path, and the output of thelaser diode is controlled positively and exactly. As the presentinvention uses the mirror 301 for deflecting the output optical beam ofthe laser diode, the optical axis of the reflected signal optical beambecomes parallel to the optical axis of the optical fiber, and thus, itbecomes possible to fix the optical fiber or optical waveguide on aplane parallel to the plane of the optical module and the opticalwaveguide 302 or optical fiber is fixed easily. Thereby, the obtainedstructure has an improved rigidity. Further, it is also possible toprovide a lens before or after the mirror 301.

[0948] Although not illustrated, it is possible to use a plurality oflaser diodes and corresponding optical waveguides or optical fibers. Inthis case, the laser diodes are arranged in a direction perpendicular tothe sheet of the drawing, and the mirror 301 extending also in thedirection perpendicular to the sheet of the drawing is used commonly bythe laser diodes forming the array.

[0949]FIG. 106 shows the block diagram of the feedback circuit thatcontrols the output of the laser diode 32 by using the divided laserbeam.

[0950] Referring to FIG. 106, the laser diode 32 is driven by a drivercircuit 161 corresponding the driver circuit 161 explained previouslywith reference to FIG. 87, and a part of the laser beam produced by thelaser diode 32 is provided to the monitoring photodetector 142P. Themonitoring photodetector 142P detects the output of the laser beam thussupplied and produces an electric output current indicative of theoutput of the laser diode 32. Further, the laser control part 162controls the laser diode 32 such that the output of the laser diode ismaintained constant.

[0951] In the construction of FIG. 106, it should be noted that thelaser diode 32 can be driven with a low drive voltage. Thereby, ordinaryCMOS circuit can be used for the driver circuit 161. Thereby, powerconsumption of the laser diode is reduce. As the wavelength band to bedetected by the monitoring photodetector 142P is in the range of 1.1-1.7μm, it is possible to use a photodiode of the InGaAs system. Further, inview of the gradual change of the output of the laser diode, it is notnecessary that the photodetector 142P has a high response speed. Thus,it is possible to use a high-sensitivity photodetector while sacrificingthe response speed.

[0952] Referring back to FIG. 106, there is formed a laser diode arrayincluding four laser diodes 32A each having the construction of FIG. 1,and the laser diode array is mounted on the mount substrate 301 of Sitogether with the drive circuit and the laser control circuit notillustrated. The laser diodes 32A has the wavelength of 1.3 μm and maybe disposed on a single chip 32 with the pitch of 200 μm.

[0953] Next, the mirror 301 is formed by using a heat conductive Sisubstrate, which is transparent to the wavelength of 1.3 μm. By usingthe Si substrate for the mount substrate 301A, it is possible to formthe mirror easily by conducting an anisotropic etching process. Thereby,a crystal surface determined with respect to the principal surface ofthe substrate 301A is formed with exact angle. The etching process maybe conducted by using an etchant such as KOH. With this, a mirrorsurface having a 45 degrees is formed. Further, an Au deposition isconducted on the mirror surface 301, and the optical waveguide 302 isformed. In such a process, it is possible to control the transmittanceof the mirror 301 by controlling the thickens of the Au film.

[0954] In the formation of the optical waveguide 302, it should be notedthat a PMMA film is formed as the core layer after the cladding layer302A is formed. By patterning the core layer thus formed, the corepattern 302B is formed. Further, the upper cladding layer 302C is formedso as to cover the core layer 302B.

[0955] In the present embodiment, the core pattern 302B is patterned soas to form a cross section of 50×50 μm. The optical waveguide layer 302thus formed is coupled with the optical fiber not illustrated, and longdistance optical telecommunication system is constructed.

[0956] For the optical waveguide layer 302, it is possible to usevarious resins such as polyimide, epoxy resin, polyurethane orpolyethylene, in addition to PMMA. Further, it is possible to provide aninorganic film such as silicon oxide film. Further, the formation of theoptical waveguide layer 302 can be made by combining the spin coatingprocess or dip coating process and a patterning process. Alternatively,it is possible to form the optical waveguide by a resin molding; processor molding process.

[0957] Further, the optical axis of the laser diode is set coincidentwith the optical axis of the optical waveguide, and the mount substrates301 and 301A are fixed. Further, a planar photodiode is mounted on themount substrate 301A in optical alignment with the laser beam divided bythe mirror 301 as the monitoring photodetector 142P. A photodiode havingan optical absorption layer of InGaAs on the InP substrate may be usedof this purpose.

[0958] Further, the output of the photodetection device 142P isconnected to the laser control unit 162 by a wire bonding process. Withthis, the feedback control circuit explained with reference to FIG. 106is obtained.

[0959] Table 11 below shows the result of evaluation of the opticaltransmission unit conducted by changing the external temperature. TABLE11 mirror transmissivitgy (%) remarks 0.1 X 0.2 X 0.3 X 0.5 Δ 0.7 Δ 1.0◯ 2.0 ◯ 5.0 ◯ 10 ◯ 20 ◯ 30 ◯ 40 ◯ 50 ◯ 60 Δ 70 X

[0960] In the experiment, the temperature is changed from 0-70° C. withthe step of 10° C., wherein Table 11 shows only the result of 20° C., asthe results for other temperatures were more or less the same as theresult of 20° C.

[0961] Referring to Table 11, it can be sent that an optical power of 10μW is detected by the monitoring photodetector 142P when the mirrortransmissivity is less than 1% for the optical power of mW level, whichis used commonly for optical telecommunication. As the change of theoptical power of the laser diode is smaller than this, no sufficientoptical energy needed for the control of the laser diode is supplied tothe photodetection device, and as a result, there is a fluctuation ofoptical output in the laser diode.

[0962] When the transmissivity exceeds 50%, on the other hand, too muchenergy of the laser output is used for controlling the output power ofthe laser diode. Thus, there occurs decrease of efficiency of theoptical telecommunication system.

[0963] From the result of Table 11, it can se seen that thetransmissivity of 2% or more but 30% or less is preferable for themirror 301. By using such a construction, it is possible to realize anoptical transmission unit capable of controlling the laser outputstably. Thereby, it is preferable that the transmissivity of the mirror301 is in the range of 1% or more but 50% or less, more preferably 2% ormore but 30% or less.

[0964] In the present embodiment, 4 laser diodes were used in the formof array. Of course, the number of the laser diode may be one, or thelaser diode may be used in the form of array including 8, 12, 16 or morelaser elements.

[0965] It is also possible to use an optical fiber for transmitting theoptical signals in place of the optical waveguide 301. When transmittinglarge amount of information over a long distance, the use of single modeoptical fiber is suited. In the case of transmitting information forshort distance with low cost, the use of plastic optical fiber (POF) issuitable. Further, there may be a possibility of using a multimodeoptical fiber in the intermediate applications.

[0966]FIG. 90 shows another embodiment of the present invention in whichthe electrode of the photodetection device is used for the mirror.

[0967] In the present embodiment, the long-wavelength laser diode 32 ofFIG. 1 is mounted on the Si mount substrate together with a drivercircuit and a laser control circuit not illustrated. In the presentembodiment, a laser diode having a laser oscillation wavelength of 1.2μm is used.

[0968] Similarly to the previous embodiments, a photodetector using aGaAsP material is used for the monitoring photodetector 142. In thepresent embodiment, the p-type electrode of the optical detectionsurface is used for the mirror 301.

[0969] More specifically, an Au film having a thickness of 300 nm andnot allowing passage of the 1.2 μm wavelength radiation is provided onthe photodiode as the electrode, and circular openings having variousdiameters in the range of 0.7-5 μm are formed. With this, atranamissivity of 5% is realized.

[0970] The photodiode 142P thus formed is mounted with an angle of 45degree with regard to the laser diode 32, and the output of thephotodiode 142P is connected to the laser control unit 162 electrically.Thereby, there is formed a feedback control system of the laser diodesimilar to the one shown in FIG. 106.

[0971] By providing a multimode optical fiber 302F having a core 302 fof 50 μm diameter and a clad of 125 μm diameter in optical alignmentwith the laser beam reelected by the mirror 301, an opticaltelecommunication system is constructed. Such an opticaltelecommunication system is simple in construction with reduced numberof parts, and it is possible to provide a compact optical transmissionmodule. Such an optical transmission module can control the laser outputstably and a reliable optical telecommunication is realized. In thepresent embodiment, it is also possible to form the mirror 301 on thesurface of the monitoring photodetection device separately to theelectrode.

[0972] Next, another embodiment of the present invention will beexplained with reference to FIG. 108.

[0973] Referring to FIG. 108, the present embodiment provides thelong-wavelength surface-emission laser diode 32 mounted on the Si mountsubstrate 301 together with the driver circuit and laser control circuitnot illustrated. In the illustrated example, four laser diodes 32A arearranged with the pitch of 200 μm, wherein each of the laser diodes hasa laser oscillation wavelength of 1.3 μm.

[0974] In the present embodiment, the edge of the optical waveguide iscut to form a 45 degree angle by a diamond blade, and the mirror 301 isformed by covering the oblique surface thus formed by an Au film.Thereby, the thickness of the Au film is controlled such that the Aufilm has a transmissivity of 3%. The optical waveguide 302 thusprocessed is coupled optically with the laser diode 32 by achievingoptical alignment, and the monitoring photodetector 142P is mounted onthe optical path of the optical beam divided by the mirror 301.According to such a construction, it is necessary to control the outputof the laser diode 32 via the control unit 162 in response to the outputof the photodetector 142. By doing so, it becomes possible to constructan optical module of simple construction and reduced number of parts.

[0975] [Twenty-fifth Embodiment]

[0976] Next, another embodiment of the present invention will bedescribed.

[0977] Conventionally, edge-emission laser diodes are used extensivelyin optical telecommunication systems. When using such an edge emissionlaser diode in combination with plural optical fibers, there is a needof achieving optical coupling for each of the laser diodes one by one.In the case of an edge-emission laser diode, there is another problem oflarge beam divergence in that the laser beam spreads rapidly due to thelarge emission angle. Further, the emission angle is different in thelateral direction and vertical direction, and the laser beam shows anelongated beam spot characterized by poor aspect ratio. Thus, in thecase of conventional edge-emission laser diode, it has been necessary toprovide a coupling lens for each of the laser diodes.

[0978] Because of these reasons, it was not possible to form the laserdiode in the form of high density array in the case of usingconventional edge-emission type laser diode.

[0979] In contrast, the present invention has enabled the constructionof a high-density laser array by using a number of surface-emissionlaser diodes commonly on a single chip monolithically. By combiningoptical fibers with the laser diode array thus formed, a large capacityoptical transmission system is realized.

[0980]FIGS. 110A and 110B show an example of an opticaltelecommunication system in which the long-wavelength laser diodes of1.1-1.7 μm are used in combination with optical fibers.

[0981] As represented in FIG. 110A, the surface-emission laser diode ofthe present invention is characterized by a narrow divergence of theoptical beam in any of the horizontal and lateral directions of the beamspot. In fact, the laser beam produced by such a surface-emission laserdiode is characterized by a circular beam cross-section.

[0982] In contrast, the laser diode of the edge emission type laserdiode produces an optical beam diverging rapidly with large emissionangle, wherein the laser beam produced by such an edge-emission layerdiode shows a beam shape characterized by an eclipse. Thus, the size ofthe laser beam spot is different in the horizontal direction verticaldirection.

[0983] Thus, by using the surface-emission laser diode of the presentinvention, it becomes possible to construct a large capacity opticaltransmission system by arranging a number of optical fibers with highdensity.

[0984] In the case of such a high-density telecommunication system, itis difficult to use coloring layer or identification ring used commonlyin ordinary optical fiber capes for identifying reach of the opticalfibers In the cable.

[0985] In the case of the surface-emission laser diode, it is possibleto form each beam emission part 32A constituting the arraysimultaneously and in high density by using the lithographic process Inthe case of handling a number of optical fibers in such a laser diodearray, it is more preferable to handle the optical fibers in the form ofbundle, rather than handing each of the optical fibers. Particularly, itis preferable to make a correlation between the optical fibers bundledin the form of a cable and the laser diodes in the array.

[0986] On the other hand, in the case the optical fibers 101 areassembled to form a bundle, it is not easy to identify the center of thebundle or individual optical fibers in the bundle.

[0987] For example, in the case the cladding 101 b of the optical fiber101X at the center of the optical fiber bundle is colored as representedin FIGS. 111A-111C, the recognition of the optical fiber is easy.

[0988] Thus, it is possible to achieve the correlation between the laserdiodes in the array and the optical fibers in the bundle by firstturning on the laser diode 32AZ at the center of the array. Thereby thislaser diode is correlated with the colored optical fiber at the centerof the bundle.

[0989] In the example of FIG. 112, it can be seen that four fibers arecolored at cladding layer. By correlating these four optical fibers withfour laser diodes, it is possible to easily establish the correspondencebetween the optical fiber and the laser diode for other optical fibers.

[0990] It should be noted that the cladding layer of an optical fiber isused merely for confining the light in the core. Thus, coloring of thecladding layer 101 b as shown in FIGS. 113A and 113B does not cause aproblem at all with regard to the information transmission, which takesplace through the core 101 a.

[0991]FIGS. 111B and 111C show a modification of FIG. 111A. In theexample of FIG. 111A, the optical fibers 101 are arranged in a closestpacking state. In the case of FIG. 111B, on the other hand, opticalfibers are arranged in a square grid. Further, FIG. 111C shows anexample of uses a ferule 101F for marinating the square grid arrangementof the optical fibers.

[0992]FIG. 114 shows an example of a single mode optical fiber. Ascompared with the optical fibers of multimode represented in FIGS. 113Aand 113C, the core diameter of the single mode optical fiber is verysmall with regard to the diameter of the cladding layer of 125 μm, andthe area of the cladding layer 101 b with respect to the core 101 a canreach 172 times as large as the core area of the multimode opticalfiber. In the case of the single mode fiber, recognition of the opticalfiber by using the color layer is facilitated further.

[0993] [Twenty-Sixth Embodiment]

[0994] Next, another embodiment of the present invention will beexplained. In the conventional laser diode, there has been a problemthat the threshold current is changed depending on the temperature. In atelecommunication system, the use of the laser diode at strictlyconstant temperature is difficult. Thus, in the case of theedge-emission laser diode, it has been practiced to detect the leakagelight emitted in the backward direction by a photodetector and theoutput off the photodetector has been used for a feed back control ofthe laser outputs such that the output of the laser diode is maintainedconstant.

[0995] In the case of the surface-emission laser diode, however, suchdetection of the leakage light is not possible. Thus, in theconventional surface-emission laser diode, it has been practiced toprovide a photodetector between the laser diode and the optical fiber orat the downstream side of the optical fiber, and the feed back controlhas been applied by using such a photodetector such that the output ofthe laser diode is maintained constant.

[0996]FIG. 115 shows an example of the I-L (current-optical output)characteristic of the long-wavelength laser diode of the presentinvention.

[0997] Referring to FIG. 115, the laser diode of the present inventionhas a feature, contrary to the conventional laser diode, in that thechange of the threshold current with temperature is very small. Only theslope of the curve is influenced with the temperature.

[0998] Thus, as long as the laser diode is driven at a constant drivevoltage, the change of the optical output is small even in the casethere is a temperature change.

[0999] For example, in the case the laser diode is driven at the drivecurrent of 6 mA in FIG. 115, the change of optical output associatedwith the temperature change from 10° C. to 70° C. is only 0.1 mW. Evenwhen the temperature is changed from 10° C. to 100° C., the change ofthe optical output is only 0.25 mW. In terms of S/N ratio, this isrespectively 26 dB and 18 dB. Thus, the laser diode of the presentinvention can provide a sufficient signal quality in the commonly usedtemperature environment of 20-70° C.

[1000] It should be noted that the drift of current of a constantcurrent source is in the order of ±2-3%. Thus, it is easy to achieve aconstant current control.

[1001] Thus, by setting the upper limit and lower limit for the opticaloutput and further setting the upper limit and lower limit for thetemperature, the present embodiment achieves the desires control of theoptical output within the foregoing upper and lower limits bycontrolling the drive current at a constant value x between a firstreference drive current a corresponding to the upper limit opticaloutput at the lower limit temperature and a second reference drivecurrent b corresponding to the lower limit optical output at the upperlimit temperature.

[1002] According to the present invention, the laser diode is driven ata constant current determined with respect to the target optical output.

[1003] [Twenty-Seventh Embodiment]

[1004] The threshold current of the laser diode increases gradually withtime and when the threshold current has exceeded a predetermined value,it is the lifetime of the laser diode.

[1005] Thus, there is a demand to avoid deterioration of signal qualityeven when the aging is in progress.

[1006] As shown in FIG. 117, for example, there is provided a halfmirror 411 in the transmission path 410 coupled to the laser diode 32,and the optical beam thus divided is detected by the photodetectiondevice 412. In this case, the intensity of the laser beam detected bythe photodetection device, in other words the monitor optical strength,is fed back to the control unit 415 and the constant current source 416.

[1007] In the illustrated example, the output of the photodetectiondevice 412 is supplied to the communication control unit 414, and thecommunication control unit 414 obtains the correction of the drivecurrent by referring to a conversion table 414A representing therelationship between the monitor optical output and the correction ofthe drive current. According to such a construction, it is possible toeliminate the effect of aging by removing the change of optical outputcaused by aging. Thereby, a practical telecommunication system isrealized.

[1008] In order to monitor for the aging or anomaly related to theaging, it is sufficient that the output of the photodetection device atthe reception end can be obtained. Thus, it is also possible to transmitthe reading of the photodetection device at the reception side in theform of data to the transmission side. For example, it is possible totransmit the reading of the photodetection device periodically or at anytime to the optical transmission side, separately to the opticaltelecommunication data. The data thus transmitted may be forwarded inthe transmission side from the communication control unit to the lasercontrol unit for correcting the drive current. By doing so, it ispossible to eliminate the fluctuation of optical output caused by aging.

[1009] [Twenty-Eighth Embodiment]

[1010] Next, a further embodiment of the present invention will bedescribed.

[1011]FIG. 118 shows an example of an optical telecommunication systemthat uses a long-wavelength laser diode 32 of the present invention.

[1012] Referring to FIG. 118, it can be seen that the system includes alaser diode module connected with an optical fiber 421, an opticalcircuit substrate 423 carrying the laser diode module 422, andelectronic substrates 423B and 423C provided above and below the opticalcircuit substrate 423A. The circuit substrates 423A-423C areaccommodated in a case 420, wherein the substrates 423A-423C form a airflow path in the case 420. Further, the case 420 is provided with a fan424A, and an air outlet is formed on the case 420 at the side oppositeto the fan 424A The electronic substrates 423B and 423C carry electroniccomponents 423 a thereon.

[1013] The fan 424A may be a compulsory air-feeding fan such as siroccofan, and supplies a cooling air inside the case 420. The cooling airflows along the space between the substrates 423B and 423C and theretakes place heat exchange between the air and the laser diode module 422as the cooling air is caused flow along the substrates. The cooling airthus exchanged heat is expelled from the air outlet. 424B.

[1014] In the illustrated example, the substrate 423B of the smallestheat generation is provided at the lower part and the substrate 423Ccausing a larger heating is provided thereabove.

[1015] It should be noted that the surface of the optical substrate 423Afacing the electronic substrate 423C is reduced with projections ordepressions so as to minimize disturbance of air flow. Meanwhile, asimilar effect is obtained also in the case in which a flat boardcarrying no electronic components is used for trhe electronic substrates423B or 423C. While the present embodiment uses only one fan 424A, it ispossible to increase the number of fans with the number of the laserdiodes provided in the case 420. It should be noted that the heating isincreased when the number of the laser diodes is increased.Alternatively, it is possible to increase the flow rate of the air. Inthis case, the area of the air outlet has to be increased.

[1016] It is also possible to use the fan 424A to pull the air insidethe case 420. Further, it is possible to provide a fan in the air intakeside and another fan in the air outlet side.

[1017] [Twenty-Ninth Embodiment]

[1018]FIG. 119 shows an example of the laser diode module that uses thelong-wavelength laser diode 32 of the present invention.

[1019] Referring to FIG. 119, it can be seen that two laser diodes 32Aare formed on the GaAs substrate 32 monolithically, and the GaAssubstrate 32 is carried by a Si mount substrate 131 having a largethermal conductivity. Further, the Si substrate 131 is carried on aceramic substrate 1363 having a still larger thermal conductivity. Theceramic substrate 136 is mounted on an optical circuit board such as thesubstrate 434A shown in FIG. 101. The ceramic substrate 136 is cooled.

[1020] Further, electric interconnection to the laser diode 32A isprovided by way of bonding wires connecting an electrode 136A on theceramic substrate 136 and the electrode on the laser diode 32A.

[1021] With such a construction, the heat generated by the laser diode32A is transferred to the GaAs substrate by thermal conduction and thento the Si substrate 131 of lower temperature. Further, the heat istransferred to the ceramic substrate 136 of lower temperature, andefficient cooling of the laser diode 32A becomes possible. It should benoted that the surface area of the ceramic substrate 136 is the largestamong the constituent parts, and thus, the ceramic substrate 136 iscooled efficiently by radiation and contact with the air.

[1022] The GaAs substrate constituting the laser diode chip of thepresent invention has a thermal conductivity of 0.54 W/cmK, while it isnoted that the thermal conductivity of Si is 1.48 W/cmRK. Further, amaterial having a larger thermal conductivity than Si such as BeO (2.72W/cmK) or diamond (9.0 W/cmK) can also be used, wherein the value of thethermal conductivity is the value at 300K.

[1023] Meanwhile, thermal transfer across tow substrates contacting witheach other can be increased by reducing the surface roughness andincreasing the intimateness. On the other hand, excessive processing ofthe surface merely invites increase of cost and the improvement of heattransfer is little. For example, the precision of surface flatnessbeyond 10 nm merely increases the cost and is not practical. Thus, thelower limit of the surface roughness should be around 10 nm.

[1024] With regard to the upper limit of surface roughness, excessivelyrough surface is disadvantageous for contacting two substrates. Thus,the inventor of the present invention conducted extensive study aboutthis matter and discovered that the surface roughness of 1000 nm or lessis sufficient for guaranteeing intimate contact and excellent heattransfer. In the experiments conducted by causing the laser diode tooscillate, it was confirmed that the heat generated by the laser diode32A is transferred from the laser diode chip 32 to the first substrate131, and to the second substrate 136, without causing accumulation ofheat. With this, it became possible to reduce the fluctuation ofthreshold current of the laser diode caused by heating. Thereby, itbecame possible to cause the laser diode to oscillate stably.

[1025]FIG. 120 show an example of the long-wavelength laser diode moduleof the present embodiment, wherein the GaAs substrate 32 carrying thelaser diode 32A is fixed upon the Si mount substrate 131 via a thermalconduction layer 137, and the Si mount substrate 131 is mounted on theceramic substrate 136 via another thermal conduction layer 138.

[1026] In the present embodiment, the contact surfaces of the substrates32, 131 and 136 are polished by using an alumina abrasive to a surfaceroughness Ra of 10-1000 nm, and any gap formed therebetween is filledwith the thermal conductive layer 137 or 138.

[1027] Here, it should be noted that the thermal conductive layer 137 or138 may be formed of an organic polymer material such as epoxy resin orsilicone resin or acryl resin dispersed with metal powders such asaluminum or gold or silver or copper The metal powders have a diameterof several nanometers to 100 nm and the resin layer is applied with athickness of 3-100 μm. The proportion of the metal powders was set to0.1-1 part with regard to 1 part of resin.

[1028]FIG. 121 is an example of a long-wavelength laser diode of thepresent embodiment and is formed of a laser module package 431 includingthe laser diode module 422 of FIG. 118 and an optical fiber 421.

[1029] At the bottom part of the package 431, there is provided acooling fin structure 431A having eight cooling fins each having arectangular cross-section, with a base length of 1 mm and a height of 3mm, and the cooling air is caused to flow along the cooling finstructure 431A.

[1030] It should be noted that the outer surface of the laser modulepackage 431 performs also as a heat-radiating member similar to theceramic substrate 136, in addition to the function of the packaging ofthe laser diode. It should be noted that the surface on which thecooling fins are provided ins not limited to the bottom surface but maybe other surface along with the cooling air is caused to flow. Further,the number and construction of the cooling fins are by no means limitedto the one illustrated.

[1031] The point of the present embodiment is to provide, in a heatradiating structure in which a first substrate including a heat sourceand a second substrate not including a heat source are stacked, suchthat the surface area of the part of the second substrate not contactingwith the first substrate is set larger than the surface area of thesecond substrate contacting with the first substrate. Any of thestructure may be used in place of the cooing fin as long the structureincreases the surface area of the second substrate.

[1032] As noted previously, in the surface-emission laser diodeoscillating at the wavelength of 1.1-1.7 μm, it is possible to form anumber of laser diodes on a common chip and is suitable for largecapacity optical telecommunication system. However, design of such aconstruction including a large number of laser diode has to be madecarefully in view of severe heating. The present embodiment provides asolution to this problem. Larger the number of the laser diodes, moreeffective the construction of the present embodiment.

[1033] [Thirtieth Embodiment]

[1034]FIGS. 122 and 123 show another embodiment of the presentinvention, wherein FIG. 122 shows the case a number of long-wavelengthsurface-emission laser diodes 32A of the present invention are used toform a one-dimensional array, while FIG. 133 show the case the laserdiodes form a two-dimensional array.

[1035] In the drawings, the laser diode elements 32A provided withshading have corresponding photodetection devices, while other elementshave no photodetection device and the output thereof is taken to theoutside.

[1036]FIG. 124 shows the construction of FIG. 122 from a differentangle. It can be seen that the laser diode elements 32A provided withthe shading is provided with the photodetection device 142P and thephotodetection device 142P interrupts the laser beam. On the other hand,the laser beam of the laser diode 32A not provided with thephotodetection device 142P is injected into the optical fiber 352 by thelens 353.

[1037] The optical output of the laser diode 32A not provided with thephotodetector 142P is calculated based on the output of thephotodetector(s) 142P located in the vicinity thereof. Based on theoutput of the photodetector 142P thus obtained, output of each of thelaser diodes 32A is controlled. The variation of output between thelaser diodes 32A is easily corrected by using a correction coefficientobtained in advance.

[1038] FIGS. 125-127 show an example of output control of the laserdiode using the construction of FIG. 124.

[1039] Referring to FIG. 125, the laser diode chip 32 includes aone-dimensional array of laser diodes 32A similar to FIG. 122. For thesake of convenience of explanation, the laser diodes 32A in FIG. 125 areprovided with numbers (0)-(6).

EXAMPLE 1

[1040] In Example 1, the out put of the photodetection device 142P thatmonitors the output of the first laser diode 32A (1) is used forcontrolling the drive currents of the laser diodes 32A (0)-(6) of FIG.125 by using the driver circuit 106 of FIG. 106 such that

[1041] I₀ is controlled based on the value of a₀O₀;

[1042] I₁ is controlled based on the value of a₁O₀;

[1043] I₂ is controlled based on the value of a₂O₀;

[1044] I₃ is controlled based on the value of a₃O₀;

[1045] I₄ is controlled based on the value of a₄O₀;

[1046] I₅ is controlled based on the value of a₅O₀; and

[1047] I₆ is controlled based on the value of a₆O₀,

[1048] wherein Ii represents the drive current of the i-th (i=0-6) laserdiode 32A and Ol represents the optical output of the i-th laser diode32A. Further, a_(i) represents a correction coefficient and isdetermined in advance by measuring the output of respective laser diodessuch that the output calculated based on the drive current is closest tothe actual laser output.

EXAMPLE 2

[1049] In Example 2, the out put of the photodetection device 142P thatmonitors the output of the first laser diode 32A (1) is used forcontrolling the drive currents of the laser diodes 32A (0)-(6) of FIG.125 by using the driver circuit 106 of FIG. 106 such that

[1050] I₀ is controlled based on the value of a₀O₀;

[1051] I₁ is controlled based on the value of a₁O₀+b₁O₃;

[1052] I₂ is controlled based on the value of a₂O₀+b₂O₃;

[1053] I₃ is controlled based on the value of a₃O₃;

[1054] I₄ is controlled based on the value of a₄O₀+b₄O₆;

[1055] I₅ is controlled based on the value of a₅O₀+b₅O₆; and

[1056] I₆ is controlled based on the value of a₆O₆.

EXAMPLE 3

[1057] In Example 3, the out put of the photodetection device 142P thatmonitors the output of the first laser diode 32A (1) is used forcontrolling the drive currents of the laser diodes 32A (00)-(44) of FIG.128 by using the driver circuit 106 of FIG. 106 such that

[1058] I₀₀ is controlled based on the value of a₀₀O₀₀;

[1059] I₀₁ is controlled based on the value of a₀₁O₀₀;

[1060] I₁₀ is controlled based on the value of a₁₀O₀₀;

[1061] I₁₁ is controlled based on the value of a₁₁O₀₀;

[1062] I₀₂ is controlled based on the value of a₀₂O₀₃;

[1063] I₀₃ is controlled based on the value of a₀₃O₀₃;

[1064] I₀₄ is controlled based on the value of a₀₄O₀₃,

[1065] . . .

[1066] I₁₂ is controlled based on the value of a₁₂O₀₃;

[1067] I₁₃ is controlled based on the value of a₁₃O₀₃;

[1068] I₁₄ is controlled based on the value of a₁₄O₀₃,

[1069] wherein a_(ij) is a correction coefficient.

EXAMPLE 4

[1070] In the example of FIG. 129, the ij-th laser diode 32A of FIG. 129is controlled such that

[1071] I₀₀ is controlled based on the value of a₀₀O₀₀;

[1072] I₀₁ is controlled based on the value of a₀₁O₀₀+b₀₁O₀₄+c₀₁O₂₂;

[1073] I₁₁ is controlled based on the value of a₁₁O₀₀+b₀₁O₂₂;

[1074] I₂₁ is controlled based on the value of a₂₁O₀₀+b₂₁O₂₂+c₂₁O₁₀;

[1075] In Example, 4, a_(ij), b_(ij) and c_(ij) are correctioncoefficients.

[1076] [Thirty-First Embodiment]

[1077] Next a further embodiment of the present invention will beexplained.

[1078] The present embodiment is related to the production control ofthe laser array module that uses the laser diode of the presentinvention and used for the long-wavelength optical telecommunication.

[1079] Here, it should be noted that the laser array module means that aplurality of surface-emission laser diodes are provided in the form of amodule and includes the case in which a number of laser diodes areformed on a single chip or the case in which a number of such chips arearranged. Further, the case of plural chips, each carrying a singlelaser diode, are arranged is also included.

[1080] In the production process of such a laser diode module, laserchip modules carrying a laser diode array are produced.

[1081] In such a production process, a predetermined number of laserdiode devices or laser diode chips are prepared and these elements arearranged to form the desired laser array module.

[1082] Thereafter, an inspection process is conducted with regard to thelaser diode modules, wherein the inspection is conducted for each of thelaser diode chips and each of the laser diodes. Upon confirmation that adesired quality is achieved for each of the devices and each of thechips, the laser array module becomes the state ready for shipping.

[1083] In the case there is detected a single device out of n devices oflaser diode chips arranged to form the laser array module is defective,the laser chip module is treated as a defective product.

[1084] On the other hand, if it is possible to treat a laser chipmodule, in which the number c of the defective laser diodes or chips iswithin a predetermined number n of good products, as being a goodproduct, the productivity of the laser array module is increasedsignificantly.

[1085] The inventor of the present invention realized the importance ofsuch a product control process and diligently studied the productioncontrol process applicable to the case of producing a product that usesa large number of elements each performing a single and same function asother elements.

[1086] Hereinafter, the present embodiment will be described withreference to FIGS. 130 and 131.

[1087] Referring to FIG. 130, the production of the laser diode or chipis conducted according to the step S1 of wafer process step, step S2 forproducing a laser diode array step, and S3 for producing a laser arraymodule. Further, there is provided an inspection step S4 for inspectingthe array or module produced by the steps S2 and S3.

[1088] In the present embodiment, the laser array module that turned outto include a defective device or defective chip is not immediatelytreated as a defective product. In the event the number c of the defectsis within a certain threshold (n−c) (n is the total number of thedevices or chips in the module), the product is shipped as the producthaving the quality of (n−c). In the process of FIG. 113, thisexamination step is conducted at the step S6. In the case the entirechips n are defective, the laser diode module is treated as a defectivedevice.

[1089] Thus, in the step S4, of FIG. 131, an inspection step S41 isconducted for evaluating the product quality for each channel CH byusing a high-frequency probe or analyzer.

[1090] The data of the product quality for each channel thus obtained isthen examined in the product examination step S42, wherein the step S42judges whether or not the required quality is satisfied for all of thelaser diodes or chips. The judgment is made with regard to the itemssuch as I-L characteristic, I-V characteristic, fiber coupling loss,pulse modulation characteristic, temperature dependence, and the like.Thereby, the laser array modules are shipped according to the gradescorresponding to the number of the properly functioning laser diodechips.

[1091] From the explanation noted above, it will be apparent that thepresent embodiment enables the use of most of the laser array modules,used for the optical telecommunication system by using thesurface-emission laser diode of 1.1-1.7 μm, according to the conditionof the module.

[1092] According to the present embodiment, therefore, the laser arraymodules are used effectively even in the case only n-c of the laserdiodes operate properly, and the loss of the laser array module duringthe production process is minimized.

[1093] [Thirty-Second Embodiment]

[1094] In the thirty-second embodiment of the present invention, thereis provided a semiconductor distributed Bragg reflector comprising anintermediate layer between two semiconductor layers of different kind,said two semiconductor layers of different kind having respective,different refractive indices, said intermediate layer having arefractive index intermediate between said refractive indices of saidtwo semiconductor layers of different kinds, wherein said intermediatelayer has a thickness such that said thickness is different in a part ofa region inside said semiconductor distributed Bragg reflector and inother regions.

[1095]FIG. 132 shows an example of the semiconductor distributed Braggreflector of the thirty second embodiment of the present invention. Itshould be noted that the semiconductor distributed Bragg reflector ofFIG. 132 is a p-type semiconductor distributed Bragg reflector having adesigned reflection wavelength of 0.98 μm and is formed on a GaAssubstrate by using an MOCVD process.

[1096] The p-type semiconductor distributed Bragg reflector of FIG. 132is produced by using trimethyl aluminum (TMA) and trimethyl gallium(TMG) for the group III source material and an arsine (AsH₃) gas is usedfor the group V material. Further, CBr₄ is used for the dopant ofp-type.

[1097] It should be noted that the semiconductor distributed Braggreflector of FIG. 132 is formed of consecutive stacking of a p-typesemiconductor distributed Bragg reflector I and a p-type semiconductordistributed Bragg reflector II, and there is provided a linearly gradedlayer between the different semiconductor layers having respective,different refractive indices and constituting the semiconductor Braggreflector, as an intermediate layer (semiconductor layer) having arefractive index intermediate of the foregoing refractive indices. Inthe linearly graded layer, it should be noted that the Al content ischanged linearly from a first composition to a second composition asrepresented in FIG. 133. Here, FIG. 133 is a diagram showing the bandenergy for the part in the vicinity of the graded layer. It should benoted that the compositional graded layer can be grown easily by usingan MOVCD process, as the AlGaAs content can be changed easily bycontrolling the supply quantity of the source materials.

[1098] Here, it should be noted that the thickness of the intermediatelayer (linearly graded layer) is changed between the one provided in thep-type semiconductor distributed Bragg reflector I and the one providedin the p-type semiconductor distributed Bragg reflector II, wherein thesemiconductor distributed Bragg reflectors I and II are stackedconsecutively in a stacking direction in the semiconductor distributedBragg reflector of the thirty-second embodiment.

[1099]FIG. 134 shows the construction of the p-type semiconductordistributed Bragg reflector I of FIG. 132, while in FIG. 135 shows theconstruction of the p-type semiconductor distributed Bragg reflector IIof FIG. 132.

[1100]FIG. 134 shows the structure for one period of the p-typesemiconductor distributed Bragg reflector I of FIG. 132. Referring toFIG. 134, it can be seen that p-AlAs is used for the low refractiveindex layer and p-GaAs is used as the high refractive index layer in theBragg reflector I. Further, an intermediate layer (p-AlGaAs linearlygraded layer) is provided between these semiconductor layers with athickness 60 nm. In the semiconductor distributed Bragg reflector ofFIG. 132, the structure of FIG. 134 is repeated four times.

[1101]FIG. 135 shows the structure for one period of the p-typesemiconductor distributed Bragg reflector II of FIG. 132. Referring toFIG. 135, it can be seen that p-AlAs is used as a low refractive indexlayer and p-GaAs is used as a high r0efractive index layer in the Braggreflector II, and an intermediate layer (p-AlGaAs linearly graded layer)of 30 nm thickness is provided between these semiconductor layers. Inthe semiconductor distributed Bragg reflector of FIG. 132, the structureof FIG. 135 is repeated for 20 times, Thus, in the thirty-secondembodiment, the thickness of the intermediate layer (compositionalgraded layer) is increased in a region I (Bragg reflector I) of thedistributed Bragg reflector as compared with other region II (Braggreflector II) in the semiconductor distributed Bragg reflector in whichthe intermediate layer (compositional gradated layer) is sandwichedbetween the first and second semiconductor layers with a refractiveindex intermediate to the first and second semiconductor layers.

[1102] Here, it should be noted that the thickness of each layer formingthe Bragg reflector is adjusted so as to satisfy the phase condition ofmultiple reflection of the distributed Bragg reflector including theintermediate layer (compositional graded layer) in each of Braggreflectors I and II. More specifically, the thickness of the p-AlAslayer in the Bragg reflector I is set to 12.3 nm, and the thickness ofthe p-GaAs layer in the Bragg reflector I is set to 20.3 nm. Further,the thickness of the p-AlAs layer in the Bragg reflector II is set to51.6 nm and the thickness of the p-GaAs layer in the Bragg reflector IIis set to 40.9 nm.

[1103] It should be noted that the p-type semiconductor distributedBragg reflector of FIG. 132 is designed such that the light is incidentfrom the side of the Bragg reflector I and the impurity dopingconcentration in the region I (Bragg reflector I) is set to about 5×10¹⁷cm⁻³ such that the impurity doping concentration becomes lower to theimpurity doping concentration in the region II (Bragg reflector II).

[1104] Thus, by setting the impurity doping concentration to be small inthe region I (Bragg reflector I), to which the light comes in and hencethe electric field strength of the light is large, the problem ofconventional technology of optical loss caused by free carrierabsorption or intra-valence band absorption can be successfully reduced.

[1105] Further, in the semiconductor distributed Bragg reflector of FIG.132, it should be noted that the thickness of the compositional gradedlayer is set to 60 nm in the region I (Bragg reflector I) where thedoping concentration is relatively low, such that the thickness becomeslarger than the thickness in the region II (Bragg reflector II).

[1106] By reducing the impurity concentration level, there arises theproblem of increase of electrical resistance because of the increasedinfluence of the potential barrier formed at the heterointerface. In thesemiconductor distributed Bragg reflector of FIG. 132, on the otherhand, it becomes possible reduce the potential barrier heightsufficiently, by increasing the thickness of the compositional gradedlayer in the region I (Bragg reflector I), in which the dopingconcentration is relatively low, to a very large value of 60 nm. Withthis, the problem of increase of device resistance, caused by theinfluence of the heterointerface, which in turn is caused as a result ofthe use of low impurity doping concentration level, is successfullyprevented. Further, as a result of the use of low doping concentration,optical loss of light is reduced, and excellent semiconductordistributed Bragg reflector is obtained with regard to optical as wellas electrical properties.

[1107] In the present embodiment, the semiconductor distributed Braggreflector is fabricated by conducting a crystal growth process on a GaAssubstrate while using a MOCVD process. However, it is possible to use adifferent crystal growth process. Further, the present embodiment uses alinearly graded layer for the intermediate layer (the semiconductorlayer having an intermediate refractive index (bandgap)) providedbetween the two semiconductor layers of different kinds having differentrefractive indices and constituting the semiconductor distributed Braggreflector. However, it is also possible to use a non-linear gradedlayer, in which the composition changes non-linear, for the intermediatelayer. Also, it is possible to use a single layer or multiple layers forthe intermediate layer, as long as the intermediate layer has adifferent refractive index.

[1108] As noted before, there is a problem in a p-type semiconductordistributed Bragg reflector that it easily causes increase of resistanceas a result of the effect of potential barrier such as spikes formed atthe semiconductor heterointerface, and this problem of increase ofresistance becomes conspicuous when the bandgap difference between thesemiconductor layers forming the heterointerface is increased or whenthe doping concentration in the vicinity of the heterointerface isreduced. Conventionally, it has been practiced to increase the dopingconcentration of the heterobarrier buffer layer provided between the twosemiconductor layers of different kind such as the composition gradedlayer. However, the use of such a high doping concentration causes theproblem of increased optical absorption and deteriorated opticalcharacteristics.

[1109] Also, it is effective to reduce the impurity concentration levelfor reducing the optical absorption, while reduction of the impurityconcentration level invites increased influence of the potential barrierat the heterointerface. In order to reduce the influence of thepotential barrier formed at the heterointerface, it is conceivable toreduce the bandgap difference for the two semiconductor layers ofdifferent kind constituting the heterointerface. However, such a measurecauses a problem in that there is caused decrease of reflectivity of thereflector and it becomes necessary to increase the number of the layersin the Bragg reflector. Further, there is caused increased penetrationof the region of large electrical field intensity into the region insidethe Bragg reflector, and it becomes necessary to increase the thicknessof the low concentration doping region further. However, such anincrease of thickness of the low refractive index layer causes problemof increase of resistance. Further, it is difficult to reduce theresistance sufficiently even in the case the bandgap is reduced, unlessthe compositional graded layer provided at the heterointerface isdesigned appropriately.

[1110] In the case a distributed Bragg reflector is used as the cavityreflector of a surface-emission laser device, and the like, it isgenerally practiced to provide an oxidation confinement layer formed byoxidation of Al(Ga)As in the Bragg reflector. Further, this oxidationconfinement layer is generally provided in a low doping region near theactive layer for increasing the effect of current confinement. It shouldbe noted that the doping concentration is reduced in the region in thevicinity of the active layer for minimizing optical absorption. In, thevicinity of the oxidation confinement layer, however, there is atendency of resistance increase also in the case the dopingconcentration is not reduced because of the reduced current path causedby concentration of electric current.

[1111] In the case of the semiconductor distributed Bragg reflector ofthe thirty-second embodiment of the present invention, on the contrary,it becomes possible to reduce the resistance of the aforementionedregion which easily undergoes a resistance increase very effectively, byincreasing the thickness of the low doping region or the thickness ofthe intermediate layer (compositional graded layer) provided in theperipheral part of the oxidation confinement layer relatively withregard to other regions.

[1112] For example, FIG. 136 shows the resistance of a semiconductordistributed Bragg reflector of four pairs designed for the 0.98 μm bandas a function of the thickness of the compositional graded layer forvarious Al contents of the low refractive index layer of the Braggreflector. Here, the vertical axis represents the resistivity(normalized resistivity) of the Bragg reflector normalized by aresistivity determined simply by a bulk resistance in the case there isno influence of the heterointerface. Thus, in FIG. 136, it can be seenthat the normalized resistivity gradually approaches to 1 with decreaseof the influence of the heterointerface (increasing of thickness of thecompositional graded layer). Here, it should be noted that a GaAs layeris used for the high refractive index layer. The doping concentration ofthe Bragg reflector is set to 5×10¹⁷ cm⁻³ throughout so as to reduce theoptical absorption.

[1113] From FIG. 136, it can be seen that, while the resistivity of thesemiconductor distributed Bragg reflector decreases by reducing thedifference of the Al content with regard to GaAs, the resistivity fallsoff much more drastically by increasing thickness of the intermediatelayer (compositional graded layer).

[1114] Thus, it will be understood that the resistance can be reducedsufficiently in the case a thick compositional graded layer is provided,without reducing the difference of Al content.

[1115]FIG. 137 is a diagram showing the relationship between thereflectivity of a p-type semiconductor distributed Bragg reflectors of 5pairs designed for the 0.98 μm band and the thickness of theintermediate layer (compositional graded layer) for various Al contentsof the low refractive index layer that constitutes the Bragg reflector.Here, it should be noted that the GaAs layer forming the high refractiveindex layer has the thickness of 69.5 nm, and the AlAs layer, theAl_(0.8)Ga_(0.2)As layer, the Al_(0.6)Ga_(0.4)As layer and theAl_(0.4)Ga_(0.6)As layer forming a low refractive index layer have thethickness of 80.2 nm, 77.5 nm, and 74.8 nm, respectively in thedistributed Bragg reflector designed for the reflection wavelength of0.98 μm in the case here is provided no compositional graded layer. Byreducing the Al content of the low refractive index layer, it can beseen from FIG. 137 that the reflectivity of the Bragg reflector fallsoff significantly Further, it can be seen from FIG. 137 that theinfluence to the reflectivity becomes smaller in the case a thickintermediate layer (compositional graded layer) having a thickness ofabout 60 nm is provided, as compared with the influence caused by thechange of the Al content in a low refractive index layer.

[1116] Thus, in the semiconductor distributed Bragg reflector of thepresent embodiment, it is possible to reduce the resistance sufficientlywhile maintaining the high reflectivity by providing a thickintermediate layer (compositional graded layer), without reducing the Alcontent of the low refractive index layer to the degree that a largeinfluence is caused on the reflectivity as in the case of theconventional technology, even in the region of low doping density.Further, because of the high reflectivity, it is possible to reduce thepenetration of light into the semiconductor distributed Bragg reflector,and the layer number of a low density layers can be reduced. Thereby,the overall resistance of the semiconductor distributed Bragg reflectoris reduced. Also, because there is little influence on the reflectivity,there is no need of increasing the number of the layers of the Braggreflector, and the problem of increase of resistance caused by increaseof the number of stacks can be eliminated effectively.

[1117] In the case of providing an oxidation confinement layer such as aselective oxidation layer to the region where the thickness of thick ofthe intermediate layer (compositional graded layer) is increased forcurrent confinement, it is possible to suppress the increase ofresistance due to the fact that the resistance is reduced in the regionwhere the current is concentrated as a result of the oxidationconfinement.

[1118] As noted above, the present embodiment provides a semiconductordistributed Bragg reflector of excellent characteristics in terms ofoptical and electrical properties, such as reduced optical absorptionloss and reduced electric resistance.

[1119] In the description heretofore, a p-type distributed Braggreflector formed of two regions, the distributed Bragg reflectors I andII, in which the thickness and doping concentration of the intermediatelayer (compositional graded layer) are changed, as the thirty-secondembodiment of the present invention. On the other hand, the p-typedistributed Bragg reflector is not limited to the one formed of such tworegions but may be formed of three or more regions in which thethickness and doping concentration of the intermediate layer arechanged.

[1120] [Thirty-Third embodiment]

[1121] In the thirty-third embodiment of the present invention, thesemiconductor Bragg reflector has the feature of the thirty-secondembodiment explained before and further has the feature that the bandgapdifference of the two different semiconductor layers of differentrefractive indices is reduced in the region where the intermediatelayer, provided between the foregoing two semiconductor layers ofdifferent kinds, has an increased thickness, as compared with the regionwhere the intermediate layer has a reduced thickness.

[1122]FIG. 138 is a diagram showing an example of the semiconductordistributed Bragg reflector of the thirty-third embodiment of thepresent invention. It should be noted that the semiconductor distributedBragg reflector of FIG. 138 is a p-type semiconductor distributed Braggreflector having a designed reflection wavelength of 0.98 μm and isconstructed on a GaAs substrate by using an MOCVD process as the crystalgrowth process.

[1123] It should be noted that the semiconductor distributed Braggreflector of FIG. 138 is formed of two regions I, II (Bragg reflectorsI, II) in which the thickness of the intermediate layer (compositionalgraded layer) is changed. With this regard, the semiconductordistributed Bragg reflector of FIG. 138 is similar to the p-typesemiconductor distributed Bragg reflector of the thirty-secondembodiment. On the other hand, the distributed Bragg reflector of thepresent embodiment is different from the distributed Bragg reflector ofthe previous embodiment in the point that the Al content of the lowrefractive index layer is changed in each of the regions I and II.

[1124]FIG. 139 shows the construction of the p-type semiconductordistributed Bragg reflector I of FIG. 138 for one period, while FIG. 140shows the construction of the p-type semiconductor distributed Braggreflector II of FIG. 138 for one period.

[1125] In the example of FIGS. 139 and 140, it should be noted thatp-Al_(0.8)Ga_(0.2)As is used for the low refractive index layer ofregion I (Bragg reflector I) as shown in FIG. 139, while p-AlAs is usedfor the low refractive index layer of region II (Bragg reflector II) asshown in FIG. 140. In each of the regions I and II, a p-GaAs layer isused for the high refractive index layer.

[1126] In the Bragg reflector I, an intermediate layer (p-AlGaAslinearly graded layer) of 60 nm thickness is provided at eachheterointerface, and an intermediate layer (p-AlGaAs linearly gradedlayer) of 30 nm thickness is provided in the Bragg reflector II at eachheterointerface.

[1127] Here, it should be noted that the thickness of each layerincluding the intermediate layer (compositional graded layer)constituting the reflector in the Bragg reflectors I and II is adjustedso as to satisfy the phase condition of multiple reflection of adistributed Bragg reflector. More specifically, the thickness of thep-Al _(0.8)Ga_(0.2)As layer in the Bragg reflector I is set to 18.0 nm,while the thickness of the p-GaAs layer is set to 11.8 nm. Further, thethickness of the p-AlAs layer in the Bragg reflector II is set to 51.6nm while the thickness of the p-GaAs layer is set to 40.9 nm.

[1128] Further, the semiconductor distributed Bragg reflector of FIG.138 is designed such that the incoming light comes upon the substratesurface located at the side of the region I (Bragg reflector I), and thedoping concentration of impurities in the region I (Bragg reflector I)is set relatively lower than the impurity doping concentration in theregion II (Bragg reflector II) such as 5×10¹⁷ cm⁻³.

[1129] In the semiconductor distributed Bragg reflector of FIGS. 138,139 and 140, it should be noted that the low refractive index layer ofthe region I (Bragg reflector I) has the composition ofp-Al_(0.8)Ga_(0.2)As. Because of the reduced Al content of the lowrefractive index layer (from AlAs to Al_(0.8)Ga_(0.2)As), it becomespossible to suppress the occurrence of potential barrier at theheterointerface.

[1130] Generally a mixed crystal of AlGaAs shows the tendency ofincreased mobility for the holes forming the carriers when the Alcontent is decreased Thus, it is possible in the Bragg reflector of thepresent invention, in which the effect of potential barrier at theheterointerface is reduced as a result of the use of thick compositionalgraded layer, to achieve the reduction of resistance effectively by wayof increasing the mobility. Thus, by reducing the Al content of the lowrefractive index layer of the low doping region to the degree that thereoccurs no remarkable decrease of the reflectivity, it is possible toobtain a Bragg reflector in which the electric resistance is reducedfurthermore.

[1131] Thus, in the thirty-third embodiment, too, it is possible toobtain a semiconductor distributed Bragg reflector having excellentoptical and electrical characteristics such as reduced opticalabsorption loss and reduced electrical resistance.

[1132] In the thirty-third embodiment, the electric resistance can bereduced further, by increasing the thickness of the intermediate layer(compositional graded layer) in a region of the semiconductordistributed Bragg reflector than in other regions and further byreducing the bandgap difference between the two semiconductor layers ofdifferent kinds constituting the semiconductor distributed Braggreflector in the foregoing region.

[1133] As mentioned before, the resistance increase at theheterointerface is caused by the potential barrier formed at such aheterointerface, and the resistance becomes higher when the bandgapdifference between the semiconductor layers constituting theheterointerface is increased. Further, the resistance increases withdecreased doping concentration of the heterointerface. In the case thedoping concentration is reduced for semiconductor layer of the Braggreflector located at the incident side of the light for reducing theoptical absorption loss, the influence of the heterointerface becomesconspicuous due to the reduced doping concentration, and the resistanceincreases easily. On the other hand, by increasing the thickness of theintermediate layer (compositional graded layer) of such a region ascompared to other regions as in the thirty-second embodiment, theelectric resistance can be reduced effectively. Thereby, the reductionof the electrical resistance can be enhanced further by reducing thebandgap difference of the semiconductor layers constituting thesemiconductor distributed Bragg reflector.

[1134] In the case the semiconductor distributed Bragg reflector is usedas the cavity reflector of a surface-emission laser diode, and the like,there are often the cases in which the oxidation confinement layerformed by oxidizing Al(Ga)As is provided in the semiconductordistributed Bragg reflector. Further, there are often the cases in whichsuch an oxidation confinement layer is provided in the low doping regionclose to the active layer for enhancing the confinement effect. In theperiphery of the oxidation confinement region, however, it is easilycaused the increase of resistance because of the reduction of thecurrent path. This problem can be caused also in the case the dopingconcentration is not reduced.

[1135] On the other hand, it is possible to reduce the electricalresistance in this region drastically in the semiconductor distributedBragg reflector of the thirty-third embodiment, by increasing thethickness of the intermediate layer (compositional graded layer) for theregion where the increase of resistance occurs very easily, such as thelow doping concentration region or peripheral region of the oxidationconfinement layer. By reducing the bandgap difference for thesemiconductor layers constituting the semiconductor distributed Braggreflector further, it becomes possible to reduce the electric resistancefurther.

[1136] For example, it becomes possible to reduce the resistance byreducing the bandgap difference of the semiconductor layers constitutingthe semiconductor distributed Bragg reflector in any of the structuresby setting the thickness of the compositional graded layer to 50 nm asshown in FIG. 136. Thereby, the electric resistance can be reducedfurther by reducing the bandgap difference of the semiconductor layers.Thus, it will be understood that reduction of the bandgap difference iseffective for further reduction of the electrical resistance. Because anAlGaAs mixed crystal has the tendency of increasing the mobility ofholes with decrease of the Al content, the abovementioned constructionof reducing the bandgap difference is effective for reducing theelectrical resistance. When the bandgap difference is reducedexcessively, on the other hand, the influence to the reflectivitybecomes conspicuous and penetration of light to the semiconductordistributed Bragg reflector becomes conspicuous. Thus, it becomespossible to obtain an excellent semiconductor distributed Braggreflector having excellent electric characteristics, by choosing the Alcontent of the low refractive index layer such that the characteristicdoes not get worse remarkably.

[1137] In the thirty-third embodiment, it should be noted that thesemiconductor distributed Bragg reflector can be formed on a GaAssubstrate by conducting a crystal growth process according to an MOCVDmethod. On the other hand, it is possible to use other crystal growthmethod. In the abovementioned example, it is noted that a linearcompositional graded layer is used as the intermediate layer providedbetween two semiconductor layers of different refractive indices andconstituting the semiconductor distributed Bragg reflector(semiconductor layer having a refractive index (bandgap) intermediate ofthe two semiconductor layers of difference refractive indexes). However,it is also possible to use a non-linear compositional graded layercharacterized by a non-linear compositional change for the intermediatelayer. Further, it is possible to form the intermediate layer in theform of single or plural layers of different refractive indices.

[1138] [Thirty-Fourth Embodiment]

[1139] The thirty-fourth embodiment of the present invention has afeature in that, in the semiconductor distributed Bragg reflector of thethirty-second or thirty-third embodiment, the thickness and dopingconcentration are changed for the plural intermediate layers(semiconductor layers) inside the semiconductor distributed Braggreflector in correspondence to the electric field strength of the light.

[1140] More specifically, the thickness is increased and dopingconcentration is decreased for the intermediate layer (semiconductorlayer) in the region of the semiconductor distributed Bragg reflectorwhere the electric field strength of the light is large in thethirty-fourth embodiment of the present invention. In the region wherethe electric field strength is weak, on the other hand, the thickness ofthe intermediate layer (semiconductor layer) is reduced and the dopingconcentration is increased.

[1141]FIG. 141 is a diagram showing an example of the semiconductordistributed Bragg reflector of the thirty-fourth embodiment of thepresent invention. It should be noted that the semiconductor distributedBragg reflector of FIG. 141 is a p-type semiconductor distributed Braggreflector having a design reflection wavelength of 0.98 μm and formed ona GaAs substrate by using an MOCVD process for the crystal growthprocess.

[1142] The semiconductor distributed Bragg reflector of FIG. 141 isformed of two regions I and II, (two Bragg reflectors I and II)characterized by different thickness for the intermediate layer(compositional graded layer), similarly to the p-type semiconductordistributed Bragg reflector of the thirty-second embodiment, exceptthat, in the thirty-fourth embodiment, the thickness and the impuritydoping concentration of the intermediate layer (compositional gradedlayer) in the region I and the thickness and the impurity concentrationof the intermediate layer (compositional graded layer) in the region IIare changed in correspondence to the electric field strength of thelight. Thereby, the thickness of each layer constituting thesemiconductor distributed Bragg reflector is adjusted so as satisfy thephase condition of multiple reflection of the semiconductor distributedBragg reflector, including the intermediate layer (compositional gradedlayer).

[1143] In more detail, the semiconductor distributed Bragg reflector ofFIG. 141 is designed such that the incoming light comes upon thesubstrate surface at the side of the region I (Bragg reflector I), andp-Al_(0.8)Ga_(0.2)As is used as the low refractive index layer andp-GaAs is used as a high refractive index layer in the region I (Braggreflector I), as shown in FIG. 142. It should be noted that FIG. 142 isthe diagram showing the construction of the semiconductor distributedBragg reflector in region I. It can be seen that the semiconductordistributed Bragg reflector I is formed of five times repetition of thefundamental structure shown in FIG. 142, wherein the fundamentalstructure is formed of one pair of p-GaAs/p-Al_(0.8)Ga_(0.2)As layersand two compositional graded layers provided adjacent to the respectivelayers. It should be noted that FIG. 142 is a diagram showing theforegoing fundamental structure repeated twice for the purpose ofillustrating the change of thickness of the compositional graded layerin the semiconductor distributed Bragg reflector of FIG. 141.

[1144]FIG. 143 is a diagram showing the construction of the region II inthe semiconductor distributed Bragg reflector of FIG. 141. As shown inFIG. 143, it can be seen that p-Al_(0.8)Ga_(0.2)As is used as the lowrefractive index layer in the region II (Bragg reflector II) and p-GaAsis used as the high refractive index layer. It should be noted that FIG.143 is the diagram showing the fundamental structure in the region II(Bragg reflector II). As can be seen in FIG. 143, there is provided anintermediate layer (compositional graded layer) of 30 nm thicknessbetween p-AlGaAs and p-GaAs layer in the region II. In FIG. 141, thefundamental structure of FIG. 143 is repeated for 20 times.

[1145] Here, it should be noted that an intermediate layer(compositional graded layer) of 30 nm thickness is provided in theregion II (Bragg reflector II) at each heterointerface.

[1146] Further, the impurity doping concentration in theAl_(0.8)Ga_(0.2)As layer and the GaAs layer of the region II (Braggreflector II) is set to about 1×10¹⁸ cm⁻³. Thus, by setting the dopingconcentration of the intermediate layer (compositional graded layer) tobe generally equal to or slightly higher than the foregoing dopingconcentration, effective reduction of resistance becomes possible.

[1147] On the other hand, the doping concentration of the intermediatelayer (compositional graded layer) in the region I (Bragg reflector I)is adjusted such that the doping concentration increases gradually fromthe surface where the electric field strength of the light is largetoward the substrate (in the direction of arrow R of FIG. 141 or 142,and in correspondence to this, the thickness of the intermediate layer(compositional graded layer) is decreased gradually in the direction ofarrow R of FIG. 141 or 142 from the surface to the substrate.

[1148] In more detail, the doping concentration at the surface side isset to 5×10¹⁷ cm⁻³ in region I (Bragg reflector I), for example, and thedoping concentration is adjusted such that it increases gradually towardthe substrate (toward the doping concentration of the region II (Braggreflector II)). Further, the thickness of the intermediate layer(compositional graded layer) is adjusted to decrease gradually from thevalue of 60 nm at the surface side toward the substrate (towardthickness of 30 nm of the Bragg reflector II).

[1149] More specifically, an intermediate layer (compositional gradedlayer) of 60 nm thickness is provided to the heterointerface of theoutermost surface of the region I (Bragg reflector I), and thep-Al_(0.8)Ga_(0.2)As layer and the p-GaAs layer that sandwiching thisintermediate layer (compositional graded layer) have respectivethickness of 18.0 nm and 11.8 nm. Further, the thickness of theintermediate layer (compositional graded layer) in the region I (Braggreflector I) is changed with the doping concentration in the directiontoward the region II (Bragg reflector II) such that the thicknessgradually decreases from the thickness of 60 nm to the thickness of 30nm, which is the thickness of the intermediate layer (compositionalgraded layer) of the region II (Bragg reflector II). Along with this,the film thickness of the p-Al_(0.8)Ga_(0.2)As layer, p-GaAs layer aregradually increased. Here, it should be noted that the thickness of thep-Al_(0.8)Ga_(0.2)As layer in the region II (Bragg reflector II) is setto 51.6 nm and the thickness of the p-GaAs layer is set to 40.9 nm.

[1150] In the semiconductor distributed Bragg reflector of thethirty-fourth embodiment, the impurity doping concentration is reducedin the region where the electric field strength of the incoming light islarge. Further, the thickness of the intermediate layer (compositionalgraded layer) is chosen so as to prevent the increase of resistance as aresult of the decrease of the impurity doping concentration (thicknessof thick of the intermediate layer is increase). As a result, it becomespossible to obtain a Bragg reflector having excellent optical andelectric characteristics such as efficiently reduced absorption loss,without increasing the resistance unnecessarily without decreasing thereflectivity in the region I (Bragg reflector I).

[1151] Thus, the semiconductor distributed Bragg reflector of thethirty-fourth embodiment includes plural intermediate layers(compositional graded layers) in the Bragg reflector with differentthickness determined according to the electric field strength of thelight incident to the semiconductor distributed Bragg reflector.Further, the doping concentration of the compositional graded layer ischanged according to the electric field strength of the light incidentto the semiconductor distributed Bragg reflector (More specifically, thethickness of the compositional graded layer is increased and the dopingconcentration is reduced in the region where the electric field strengthis large. The thickness of the compositional graded layer is decreasedand the doping concentration is increased in the region where theelectric field strength is weak). With this, it becomes possible toreduce the optical absorption loss efficiency with the electric fieldstrength inside the semiconductor distributed Bragg reflector, togetherwith the resistance.

[1152] In a semiconductor distributed Bragg reflector, the thickness ofeach semiconductor layer is chosen so as to satisfy the phase conditionin which the light waves strengthen with each other, such that thereflection waves strengthen with other as a result of multiplereflection of the light caused by the difference of refractive index ofthe semiconductor layers. Here, it should be noted that the reflectionof light wave is caused not only by the surface reflection as in thecase of simple reflector but undergoes multiple reflections as itpenetrates inside the semiconductor distributed Bragg reflector.Thereby, the electric field strength of light is stronger at theincident surface.

[1153] Thus, it becomes possible to reduce the optical absorption lossand the electric resistance effectively by reducing the dopingconcentration of the semiconductor distributed Bragg reflector in theregion where the electric field strength of the light is large accordingto the electric field of the light and by determining the thickness ofthe compositional graded layer with the doping concentration such thatsufficiently low resistance is achieved. By determining the impuritydoping concentration according to the electric field strength of thelight as such, the problem of unnecessarily decreasing the dopingconcentration in the low doping concentration region provided forreducing the increase of optical absorption is eliminated, and itbecomes possible to eliminate unnecessary increase of resistance ordecrease of the reflectivity caused by providing a compositional gradedlayer.

[1154] However, it is important also in the thirty-fourth embodiment toprovide a sufficiently thick compositional graded layer in the regionclose to the surface where the electric field strength of the light islarge in view of the need of reducing the doping concentrationsufficiently.

[1155] In the case of using the semiconductor distributed Braggreflector as the cavity reflector of a surface-emission laser diode, andthe like, there are many cases in which the oxidation confinement layerformed by oxidizing Al(Ga)As is provided in the Bragg reflector.Further, such an oxidation confinement layer is tend to be provided inthe low density doping region close to the active layer for enhancingthe confinement effect. On the other hand, it is easily caused increaseof resistance as a result of the current path becoming small in theperiphery of the oxidation confinement region.

[1156] Thus, there is a problem that there is caused increase ofresistance in the vicinity of the oxidation confinement layer due to thefact that the current path is reduced as a result of the confinement anddue to the fact that the doping density is reduced. When the thicknessof the compositional graded layer is increased sufficiently in such aregion as compared with other regions as in the case of thethirty-fourth embodiment, on the other hand, it becomes possible tosuppress the increase of resistance due to the fact that the resistanceof the region where there occurs current concentration as a result ofthe current confinement, is reduced sufficiently.

[1157] By increasing the thickness of the intermediate layer(compositional graded layer) in the region of low doping concentrationor in the region in which the resistance easily increases such as theperipheral region of the oxidation confinement layer as noted above, theresistance of the Bragg reflector can be reduced very effectively.Accordingly, in this thirty-fourth embodiment of the present invention,an excellent semiconductor distributed Bragg reflector having excellentoptical and electrical characteristic is obtained.

[1158] In the thirty-fourth embodiment, a semiconductor distributedBragg reflector was explained for the case the thickness of theintermediate layer is gradually decreased and the doping concentrationof the intermediate layer is gradually increased from the incident sideof the light with the electric field strength of the light as an exampleof the semiconductor distributed Bragg reflector in which the thicknessand the doping concentration are changed. In the semiconductordistributed Bragg reflector of the thirty-second and thirty-thirdembodiments formed of two regions of different thickness for theintermediate layer, it is noted that the doping concentration is reducedin the region where the intermediate layer has an increased thicknessand the doping concentration is increased in the region where theintermediate layer has a reduced thickness. Even in these cases, it ispossible to reduce the optical absorption loss and electrical resistanceeffectively.

[1159] In the thirty-fourth embodiment, it should be noted that thesemiconductor distributed Bragg reflector is formed on a GaAs substrateby carrying out the crystal growth by an MOCVD process. However, othergrowth process may also be used. Further, while the foregoing exampleuses a linear compositional graded layer for the intermediate layerbetween the two semiconductor layers of that different refractiveindices (semiconductor layer having a refractive index (bandgap)intermediate of the refractive indices of the two semiconductor layersof different refractive indices) and constituting the semiconductordistributed Bragg reflector, it is also possible to use a non-linearcompositional graded layer in which the refractive index changesnon-linear for the intermediate layer. Further, it is possible to formthe intermediate layer from single or plural layers having differentrefractive indices.

[1160] [Thirty-Fifth Embodiment]

[1161] In a thirty-fifth embodiment of the present invention, thesemiconductor distributed Bragg reflector has a designed reflectionwavelength longer than 1.1 μm in the semiconductor distributed Braggreflectors of the any of thirty-second through thirty-fourth embodiment.

[1162]FIG. 144 is a diagram showing an example of the semiconductordistributed Bragg reflector according to a thirty-fifth embodiment ofthe present invention. It should be noted that the semiconductordistributed Bragg reflector of FIG. 144 is a p-type semiconductordistributed Bragg reflector having a design reflection wavelength of 1.3μm band and formed on the GaAs substrate by using an MOCVD process asthe crystal growth process.

[1163] Further, it should be noted that the semiconductor distributedBragg reflector of FIG. 144 is formed of two regions I and IIcharacterized by different thickness of the intermediate layer(compositional graded layer), similarly to the p-type semiconductordistributed Bragg reflector of the thirty-second embodiment.

[1164]FIG. 145 shows one period of the Bragg reflector I of FIG. 144.Referring to FIG. 145, it should be noted that p-Al_(0.8)Ga_(0.2)As isused as the low refractive index layer and p-GaAs is used as the highrefractive index layer in the p-type semiconductor distributed Braggreflector I. Further, an intermediate layer (p-AlGaAs linearcompositional graded layer) of 80 nm thickness is provided to each ofthese semiconductor layers. In the semiconductor distributed Braggreflector of FIG. 144, the construction of FIG. 145 is repeated fourtimes.

[1165] Similarly, FIG. 146 shows one period of the stacks in the p-typesemiconductor distributed Bragg reflector II of FIG. 144. In the Braggreflector II, p-AlAs is used for low refractive index layer and p-GaAsis used for the high refractive index layer. Further, an intermediatelayer (p-AlGaAs linear compositional graded layer) of 50 nm thickness isprovided to each of the interfaces of these semiconductor layers, andthe semiconductor distributed Bragg reflector of FIG. 144 is repeatedtwenty times in the construction FIG. 146.

[1166] The semiconductor distributed Bragg reflector of FIG. 144 isdesigned such that incident light comes from the side of the region I(Bragg reflector I) where there exists a substrate surface, and theimpurity doping concentration in the region I is set to be lower thanthe doping concentration in the region II, such as 5×10¹⁷ cm⁻³.

[1167] It should be noted that the design reflection wavelength of thesemiconductor distributed Bragg reflector of FIG. 144 is set to the longwavelength band of 1.3 μm, which is longer than the design wavelength ofthe conventional Bragg reflector of the 0.98 μm band. In such a longwavelength band, it is known that there occurs remarkable opticalabsorption caused by intra-valence band transition. For example,reference should be made to IEEE J. Quantum Electron, Vol. 33, No. 8,1997, pp. 1369. This reference describes that the optical absorptioncoefficient of GaAs to a light of the 1.3 μm band is about twice aslarge as in the case of the 0.98 μm band and that the optical absorptioncoefficient becomes about three times as large as in the case of the0.98 μm band for the light of the 1.5 μm band. Thus, the effect ofoptical absorption becomes very conspicuous for the light of thewavelength longer than 1.1 μm, and it becomes very important to reducethe optical absorption for realizing a highly efficient semiconductordistributed Bragg reflector.

[1168] In the semiconductor distributed Bragg reflector of FIG. 144, thedoping concentration of the region I constituting the incident side ofthe light is set to relatively low concentration, and the opticalabsorption losses is reduced in this region. Further, because of theincreased reflection wavelength, the thickness of the layersconstituting the semiconductor distributed Bragg reflector is set largerthan the case of the conventional surface-emission laser of the 0.98 μmband. Because of this, the influence to the decrease of the reflectivityis greatly reduced even in the case a thick intermediate layer(compositional graded layer) is provided. Further, as a result of use ofthe thick intermediate layer (compositional graded layer), the effect ofsmoothing the potential barrier at the heterointerface is enhancedsignificantly. Thus, the effect of reducing the electric resistance bythe intermediate layer (compositional graded layer) is significant evenin the case the bandgap difference between the two differentsemiconductor layers constituting the semiconductor distributed Braggreflector is large. Thus, the penetration of the light into thesemiconductor distributed Bragg reflector is suppressed effectively, andit becomes possible to reduce the thickness of the low doping region.Further, the number of the layers in the semiconductor distributed Braggreflector can be reduced and the electrical resistance is suppressedeffectively.

[1169] Thus, in the thirty-fifth embodiment, it is possible to obtain asemiconductor distributed Bragg reflector showing excellentcharacteristics such as reduced optical absorption losses and reducedelectrical resistance in the long-wavelength band.

[1170] In more detail, the intra-valence band absorption, which becomesthe major cause of the optical absorption in a p-type semiconductordistributed Bragg reflector, is caused as a result of the electronsabsorbing the light and causing transition from the spin orbitalsplit-off band in the valence band to the heavy hole band or and lighthole band. Because of the small difference of energy between theselevels, the absorption becomes conspicuousness for the light having along wavelength. Thus, the laser devices formed on a GaAs substrateshows a conspicuous optical absorption loss in the wavelength band of1.3 μm or 1.5 μm, which is important for optical telecommunication, andthe like, as compared with the light of 0.85 μm band or 0.98 μm band.Thus, it is difficult to obtain excellent p-type semiconductordistributed Bragg reflector of the characteristics of low resistance andsimultaneously small optical absorption by the conventional technologybecause of the large absorption coefficient in the p-type semiconductordistributed Bragg reflector.

[1171] In the construction of any of the thirty-second throughthirty-fourth embodiments of the present invention, on the other hand,it is possible to reduce the optical absorption of in region showingconspicuous optical absorption, in other words the region inside thesemiconductor distributed Bragg reflector characterized by largestrength of the light, without increasing the resistance, as mentionedbefore. In the case the semiconductor distributed Bragg reflector of thethirty-fifth embodiment is constructed to have the reflection wavelengthof the long wave even from the 1.1 ,,m while using a structure notprovided with the compositional graded layer of the 1.3 μm band, itshould be noted that the GaAs layer used for the high refractive indexlayer and the AlAs layer used for the low refractive index layer havelarge thickness of 95.2 nm and 111.6 nm respectively as a result of theincrease of the design wavelength. Longer the reflection wavelength, theproportion of the compositional graded layer in the semiconductordistributed Bragg reflector is reduced, and the effect on thereflectivity is reduced correspondingly. This means that it becomespossible to provide a thicker compositional graded layer than in thecase of the conventional semiconductor distributed Bragg reflector ofthe 0.85 μm band or the 0.98 μm band while maintaining the reflectivity.Thicker the compositional graded layer, the effect of smoothing thepotential barrier at the heterointerface is high increased, and becomesof this, it becomes possible to reduce the electric resistancesufficiently.

[1172]FIG. 147 is a diagram showing the relationship between thereflectivity and the thickness of the compositional graded layer for thesemiconductor distributed Bragg reflectors of 5 pairs and having thereflection wavelength of 1.3 μm, for various Al contents of the lowrefractive index layer. In FIG. 147, it should be noted that GaAs isused as the high refractive index layer of the semiconductor distributedBragg reflector. On the other hand, the thickness of the AlAs layer, theAl_(0.8)Ga_(0.2)As layer, the Al_(0.6)Ga_(0.4)As layer and theAl_(0.4)Ga_(0.6)As layer are set respectively to 111.6 nm, 108.2 nm,104.8 nm and 101.5 nm for the case the composition gradated layer is notprovided, Further, the GaAs layer used for the high refractive indexlayer has the thickness of 95.2 nm as noted above. Referring to FIG.147, it can be seen that the influence to the reflectivity is relativelysmall even in the case a thick compositional graded layer is provided asin the case of the semiconductor distributed Bragg reflector of the 0.98μm band.

[1173] Thus, in such a semiconductor distributed Bragg reflector havinga reflection wavelength in the long wavelength band, the influence ofthe compositional graded layer to the reflectivity is reduced ascompared with the conventional semiconductor distributed Bragg reflectorof the 0.98 μm band, and it become possible to maintain a highreflectivity in the lower doping region and the penetration of lightinto the semiconductor distributed Bragg reflector is easily reduced.

[1174] Thus, it becomes possible in the semiconductor distributed Braggreflector of the thirty-fifth embodiment to reduces the number of thelayers of the low doping region. Because the Bragg reflector as a wholehas low resistance and the reflectivity in the low doping region issufficiently high, there is no need of increasing the number of thelayers in the semiconductor distributed Bragg reflector, and the overallresistance of the Bragg reflector can be suppressed similarly.

[1175] Thus, the effect of decrease of the resistance appearsparticularly conspicuously in the semiconductor distributed Braggreflector having the wavelength of the 1.1 μm band or longer.

[1176] As noted before with reference to the thirty-second throughthirty-fourth embodiments, there are many cases in which an oxidationconfinement layer, formed by oxidizing a Al(Ga)As layer, is provided inthe Bragg reflector when the Bragg reflector is used as the cavityreflector of a surface-emission laser diode, and the like. Further,there are many cases in which the oxidation confinement layer isprovided in the low doping region near the active layer in order toenhance the confinement effect. By increasing the thickness of thecompositional graded layer in the periphery of the oxidation confinementlayer as compared with other regions, it becomes possible to suppressthe increase of the resistance because of the fact that the resistanceof the region where there occurs current concentration is sufficientlyreduced.

[1177] As noted above, it is possible to obtain a semiconductordistributed Bragg reflector having a low resistance and small opticalabsorption loss simultaneously in the semiconductor distributed Braggreflector of the type in which an intermediate layer is provided betweentwo semiconductor layers of different refractive indices with arefractive index intermediate of the two semiconductor layers withoutsacrificing the reflectivity, by setting the thickness of theintermediate layer such that thickness of the intermediate layer isdifferent in a region of the semiconductor distributed Bragg reflectorfrom other regions of the semiconductor distributed Bragg reflector.

[1178] As noted above, the thirty-fifth embodiment of the presentinvention provides a semiconductor distributed Bragg reflector showingexcellent optical and electrical characteristics such as sufficientlylow optical absorption loss and sufficiently low electric resistance inthe long-wavelength band (reflection wavelength longer than 1.1 μm).

[1179] In the thirty-fifth embodiment, the semiconductor distributedBragg reflector was formed on a GaAs substrate by carrying out a crystalgrowth by the MOCVD process. However, it is possible to use other growthprocesses such as MBE process, and the like. While a linearcompositional graded layer is used in an abovementioned for theintermediate layer provided between the two semiconductor layers ofdifferent refractive indices and constituting the semiconductordistributed Bragg reflector (semiconductor layer having an intermediaterefractive index (bandgap) of the refractive indices of the foregoingtwo semiconductor layers of different refractive indices), it is alsopossible to use a non-linear compositional graded layer in which thecomposition changed non-linear for the intermediate layer.

[1180] [Thirty-Sixth Embodiment]

[1181] By using the semiconductor distributed Bragg reflectors of any ofthe first through thirty-fifth embodiments explained above, it ispossible to construct a surface-emission laser diode.

[1182] Thus, the thirty-sixth embodiment of the present inventionprovides a surface-emission laser diode that uses the semiconductordistributed Bragg reflectors of any of the first through thirty-fifthembodiments for the cavity reflector.

[1183]FIG. 148 is a diagram showing an example of the surface-emissionlaser diode of the thirty-sixth embodiment.

[1184] The surface-emission laser diode of FIG. 148 is asurface-emission laser device of the 1.3 μm band and has an active layerof GaInNAs. The laser diode is fabricated by carrying out a crystalgrowth by an MOCVD process using trimethyl aluminum (TMA), trimethylgallium (TMG), trimethyl indium (TMI) and an arsine (AsH₃) gas as thesource materials. Further, dimethylhydrazine (DMHy) is used for thenitrogen source material when forming the active layer. Further, H₂Sewas used for the n-type dopant and CBr₄ was used for the p-type dopant.

[1185] Thus, the surface-emission laser diode of FIG. 148 is formed onan n-GaAs substrate after forming an n-GaAs buffer layer, byconsecutively forming an n-type semiconductor distributed Braggreflector including 36 pairs of AlAs/GaAs, a GaAs cavity spacer layer, aGaInNAs/GaAs multiple quantum well structure (active layer), a GaAscavity spacer layer, and a p-type semiconductor distributed Braggreflector.

[1186] Here, it should be noted that the p-type semiconductordistributed Bragg reflector is formed of a p-type semiconductordistributed Bragg reflector I (region I) including therein four pairs ofAl_(0.8)Ga_(0.2)As/GaAs and a p-type semiconductor distributed Braggreflector II (region II) including therein 18 pairs ofAl_(0.8)Ga_(0.2)As/GaAs. Further, doping is achieved for the Braggreflector I (region I) located near the active layer where the laseroscillation takes place and the light electric field strength is large,to the level of 5×10¹⁷ cm⁻³, which is relatively lower than the dopingconcentration of the Bragg reflector II of 1×10¹⁸ cm⁻³, such that theabsorption loss of the oscillation light is reduced.

[1187] Further, there is provided a linear compositional graded layer(intermediate layer) of 80 nm thickness that changes the Al contentlinearly from the side of a semiconductor layer to the side of anothersemiconductor layer, to each of the heterointerfaces of the p-typesemiconductor distributed Bragg reflector I (region I). Further, alinear compositional graded layer (intermediate layer) of 50 nmthickness is provided to each of the heterointerfaces of the p-typesemiconductor distributed Bragg reflector II (region II).

[1188] In the surface-emission laser diode of the long wavelength suchas the 1.3 μm band, it is possible to provide a thick compositionalgraded layer (intermediate layer) like this, without decreasing thereflectivity significantly.

[1189] Also, in the surface-emission laser diode of FIG. 148, there isprovided an AlAs selective oxidation layer of 30 nm thickness in thep-type semiconductor distributed Bragg reflector I (region I) at theinterface at the location of first pair from the active layer. Also, theGaAs layer at the outermost surface of the p-type semiconductordistributed Bragg reflector has an increased doping concentration andfunctions also as a contact layer.

[1190] The thickness of each layer constituting the p-type semiconductordistributed Bragg reflector and the n-type semiconductor distributedBragg reflector is adjusted so as to satisfy the phase condition ofmultiple reflection of the distributed Bragg reflector, including thecompositional graded layer (intermediate layer), similarly to thethirty-second embodiment. Further, the thickness of theAl_(0.8)Ga_(0.2)As layer adjacent to the AlAs selective oxidation layeris adjusted similarly. Thus, there is formed a λ cavity. The activelayer is formed at the center of the λ cavity where is placing theanti-node of standing wave of lasing light.

[1191] The surface-emission laser diode of FIG. 148 is produced, afterthe stacked structure (device stacking film) noted above is formed, asfollows.

[1192] After the formation of the aforementioned stacked structure, anetching process is applied to each of the layers starting from thesurface of the p-type GaAs contact layer up to the midway of the GaAscavity spacer layer adjacent to the n-type semiconductor distributedBragg reflector, by carrying out known photolithographic process and dryetching process so as to leaving a region that becomes the deviceregion. Thereby, a mesa region is formed as the device region in thesquare form having the size of 30 ,,m×30 ,,m.

[1193] Next, a heating process is conducted in the atmosphere that wasobtained by bubbling a heated pure water with nitrogen gas, andselective oxidation process is conducted laterally from the etchingsidewall of the AlAs selective oxidation layer toward the interior ofthe device, and a current confinement structure is formed. Here, theregion that becomes a current path is formed to have the size of 5 ,,m×5,,m.

[1194] Next, the mesa part is buried by an insulation resin such aspolyimide, and the like, and a p-type electrode having an opening forthe laser output is formed at the top part of the device by using anevaporation process of an electrode material and a lift off process.Further, by forming an n-type electrode at the rear surface of the GaAssubstrate, the surface-emission laser diode of FIG. 148 is obtained.

[1195] In the surface-emission laser diode of FIG. 148, it should benoted that the doping concentration of the p-type semiconductordistributed Bragg reflector is reduced particularly in the region nearactive layer where the intensity of the oscillation light is large. Withthis, the optical absorption loss is reduced and the slope efficiency ofthe device is improved. Thereby, the oscillation threshold current isreduced. Further, by increasing the thickness of the intermediate layer(compositional graded layer) in the p-type Bragg reflector I (region I)as compared with the intermediate layer (compositional graded layer) inthe p-type Bragg reflector II (region II), the potential barrier at theheterointerface is sufficiently smoothed in spite of the fact that thedoping concentration is reduced, and increase of resistance andoperational voltage is prevented successfully. Thus, there occurs noincrease of heat generation for the device, and there occurs nodegradation of output as a result of the heat generation. Because of thedecrease of the optical absorption loss, the device can provide higheroutput as compared with the conventional case.

[1196] It should be noted that the surface-emission laser diode of FIG.148 uses GaInNAs for the active layer. Thus, it is possible to form asurface-emission laser diode oscillating at the wavelength of 1.3 μm byusing the high-performance semiconductor distributed Bragg reflectorformed of the Al(Ga)As/GaAs layers on the GaAs substrate. As for theGaInNAs mixed crystal, it should be noted that there exists a largeconduction band discontinuity with regard to the GaAs cavity spacerlayer, and the stable laser oscillation achieved up to high temperaturebecause of the increased confinement effect of the electrons in theactive layer. As the 1.3 μm band corresponds to the zero dispersion bandof a silica single mode fiber, a high speed communication becomespossible by using a single mode fiber. Thus, the surface-emission laserdiode of FIG. 148 can realize a high speed telecommunication systemeasily by being combined with a silica single mode fiber.

[1197] In the examples noted above, the surface-emission laser diode wasexplained as being produced by conducting a crystal growth process on ann-type semiconductor substrate. However, it is also possible to producea surface-emission laser diode by conducting a crystal growth process ona p-type semiconductor substrate.

[1198]FIG. 149 shows a surface-emission laser diode produced byconducting a crystal growth process on a p-type semiconductor substrate.The surface-emission laser diode of FIG. 149 is produced by conductingthe crystal growth process on the p-GaAs substrate by an MOCVD processsimilarly to the case of the surface-emission laser diode of FIG. 148.

[1199] Thus, the surface-emission laser diode of FIG. 149 is produced,after conducting the crystal growth of the p-GaAs buffer layer on thep-GaAs substrate, by conducting the crystal growth of the p-typesemiconductor distributed Bragg reflector I (region II) such that anAl_(0.8)Ga_(0.2)As/GaAs structure is repeated 32 times. Next, thecrystal growth of the p-type semiconductor distributed Bragg reflector I(region I) is conducted by repeating the Al_(0.5)Ga_(0.5)As/GaAsstructure for five times. Further, the crystal growth of the GaAs cavityspacer layer, the GaInNAs/GaAs multiple quantum well active layer, theGaAs spacer layer are conducted, and the crystal growth of the n-typesemiconductor distributed Bragg reflector is conducted by repeating theAl_(0.8)Ga_(0.2)As/GaAs structure for 24 times.

[1200] In the surface-emission laser diode of FIG. 149, there isprovided a linear compositional graded layer (intermediate layer) of 50nm thickness at each heterointerface of the p-type semiconductordistributed Bragg reflector II (region II) for reducing the electricresistance similarly to the surface-emission laser diode of FIG. 148.Further, there is provided a linear compositional graded layer(intermediate layer) of 80 nm thickness at each heterointerface of thep-type semiconductor distributed Bragg reflector I (region I), tosatisfy the multiple reflection condition of the semiconductordistributed Bragg reflector. Further, an AlAs selective oxidation layeris provided to the heterointerface of Al_(0.8)Ga_(0.2)As/GaAs nearest tothe active layer so as to satisfy the phase condition.

[1201] The surface-emission laser diode of FIG. 149 is subjected to thedry etching process, selective oxidation process, burying process by aninsulating resin, and electrode formation process after the crystalgrowth process, similarly to the surface-emission laser diode of FIG.148. In order to cause oxidation in the AlAs selective oxidation layerformed in the p-type semiconductor distributed Bragg reflector, itshould be noted that the dry etching process is carried out such thatthe etching reaches the midway of the p-type semiconductor distributedBragg reflector.

[1202] In the surface-emission laser diode of FIG. 149, too, theoscillation (lasing) threshold current is reduced and the slopeefficiency is improved as a result of the decrease of the opticalabsorption loss of the p-type semiconductor distributed Bragg reflector,similarly to the surface-emission laser diode of FIG. 148. Because ofthe elimination of increase of the device resistance, the operationalvoltage is reduced and high output power becomes possible.

[1203] Also, the surface-emission laser diode of FIG. 149 can realize ahigh speed telecommunication system easily by being combined with asilica single mode fiber.

[1204] Thus, in the surface-emission laser diode of the thirty-sixthembodiment, it becomes possible to reduce the absorption loss of thelight and improve the slope efficiency and reduce the oscillationthreshold current by reducing the doping concentration of the p-typesemiconductor distributed Bragg reflector in the region close to theactive layer where the electric field intensity of the oscillation lightis large, with respect region the region away from the active layer, andhence, the electric field intensity of the oscillation light isrelatively small.

[1205] In the present invention, the potential barrier at theheterointerface in the low doping region of a p-type Bragg reflector issmoothed sufficiently by increasing the thickness of the compositionalgraded layer in the low doping region as compared with the high dopingregion. Thereby, it becomes possible to reduce the optical absorptionloss without increasing the resistance. Thereby, it should be noted thatthe influence caused in the reflectivity is small even when thethickness of the compositional graded layer is increased sufficiently asin the case of FIG. 137. Thus, it is possible to provide a sufficientlythick compositional graded layer. Thus, it is no longer necessary todecrease the Al content significantly in the low concentration region asis practiced in the conventional art for reducing the electricalresistance, and it is possible to maintain high reflectivity in theregion where the doping concentration is low. Thus, diffusion of lightinto the Bragg reflector is reduced and the thickness of the low dopingregion is reduced, and as a result, increase of the resistance issuccessfully eliminated. Further, the stacking number of the Braggreflector is reduced and the increase of the resistance is successfullyprevented.

[1206] In a surface-emission laser diode, there are many cases that anoxidation confinement layer formed by oxidizing Al(Ga)As is provided inthe semiconductor distributed Bragg reflector. Further, there are manycases in which the oxidation confinement layer is provided in the lowdoping region close to the active layer in order to increase theconfinement effect In the periphery of the oxidation confinement layer,there is a problem that resistance increases easily even in the case thedoping concentration is not reduced due to the decrease of the currentpath by current confinement.

[1207] On the other hand, by increasing the thickness of thecompositional graded layer in the low doping concentration region or theperipheral region of the oxidation confinement layer where there occursincrease of the resistance easily with respect to other regions, it ispossible to decrease the resistance of the region very effectively as inthe example of the semiconductor distributed Bragg reflector of thepresent invention.

[1208] Thus, it becomes possible to reduce the operation voltage anddevice heat generation as compared with a conventional device, and itbecomes possible to obtain a surface-emission laser diode characterizedby reduced oscillation threshold current and improved slope efficiencyand increased saturation output power as a result of reduced opticalabsorption loss.

[1209] Thus, the surface-emission laser diode of the thirty-sixthembodiment has the advantageous characteristics in that the opticalabsorption loss of the oscillation light inside the surface-emissionlaser diode is reduced, the slope efficiency is improved, theoscillation threshold current is reduced, a higher output operation isenabled, and the electric power transformation efficiency is improved,without increasing the device resistance.

[1210] In the surface-emission laser diode of the thirty-sixthembodiment, it is possible to use any or all of Ga and In for the groupIII material of the active layer and any or all of As, N and Sb for thegroup v material of the active layer.

[1211] The active layer formed of these materials can be grown on theGaAs substrate by a crystal growth process, and it is possible obtain asurface-emission laser diode that uses a DBR of the AlGaAs materialsystem, which has excellent characteristics with regard to reflectivity,thermal conductivity, process controllability (crystal growth, selectiveoxidation of the Al(Ga)As mixed crystal, and the like). Further, byusing these materials for the active layer, it is possible to obtain theoscillation light not only in the 0.85 μm band, 0.98 μm band but also inthe longer wavelength band, including the 1.1 μm band, 1.3 μm band and1.5 μm band, which are important in the optical-fiber telecommunication.

[1212] By combining the surface-emission laser diode of the 1.3 μm bandwith a silica single mode optical fiber, for example, it is possible torealize high speed optical telecommunication. Further, it is possible torealize a large capacity communication by using the technology of DWDMby the device of the 1.5 μm band.

[1213] Here, it should especially be noted that, among the variousmaterials of the active layer noted above, the GaInN(Sb)As mixed crystalmaterial can realize the oscillation wavelength of 1.1 μm or longer whenused for active layer material that mentioned above. Further, aGaInN(Sb)As layer provides a large band discontinuity at the conductionband with respect to the GaAs layer that acts as a carriers confinementlayer. Thus, the overflow of electrons is reduced and it becomespossible to obtain a stable laser oscillation up to high temperatures.

[1214] In addition to these, the surface-emission laser diode of thepresent invention has the advantageous feature in that the slopeefficiency is improved because of the smaller optical absorption lossand lower resistance as compared with a conventional device as notedabove, and can reduce the oscillation threshold current. Further,because of the low resistance, the saturation output power is increasedand a high output power is obtained. Further, the electric powertransformation efficiency is high, and the electric power consumption islow. As noted above, it is possible to provide a surface-emission laserdiode suitable for optical telecommunication and optical transmission.

[1215] In each of the aforementioned embodiments, explanation was madefor the case an AlGaAs mixed crystal is used as the material of thesemiconductor distributed Bragg reflector. On the other hand, it is alsopossible to use a GaInP mixed crystal for this purpose. A GaInP mixedcrystal can achieve lattice matching to the GaAs substrate and has alarge bandgap as compared with a GaAs compound semiconductor (refractiveindex is smaller as compared with a GaAs compound semiconductors). Thus,this can be used for the low refractive index layer in place of theAlGaAs mixed crystal. Further, the GaInP mixed crystal semiconductor hasan etching selectivity with respect to the AlGaAs mixed crystalsemiconductor layer in a wet etching process, and it can be used for theetching stopper layer in the case of forming mesa by a wet etchingprocess. Thereby, the controllability of the wet etching process isimproved at the time of forming a surface-emission laser by using such aprocess.

[1216] [Thirty-Seventh Embodiment]

[1217] A thirty-seventh embodiment of the present invention is asurface-emission laser array formed of the surface-emission laser diodeof the thirty-sixth embodiment.

[1218]FIG. 150 is a diagram showing an example of the surface-emissionlaser array of the thirty-seventh embodiment. The surface-emission laserarray of FIG. 150 is a monolithic laser array in which thesurface-emission laser diode of the thirty-sixth embodiment isintegrated two-dimensionally in a 3×3 array. In order to drive thesurface-emission laser elements independently, there are provided pelectrode wirings individually in the surface-emission laser array ofFIG. 150. It should be noted that the surface-emission laser array ofFIG. 150 is produced with a procedure and method similar to those of thethirty-sixth embodiment.

[1219] In the surface-emission laser array of FIG. 150, the individualsurface-emission laser elements constituting the surface-emission laserarray have the feature of low optical absorption loss and lowresistance. In view of the fact that the electric power transformationefficiency is improved further when the laser diode elements areintegrated in the form of array, the high surface-emission laser arraycan provide further improved efficiency.

[1220] Thus, in the surface-emission laser diode of the presentinvention, the slope efficiency is increased, the oscillation thresholdcurrent is reduced and the electric power transformation efficiency isincreased as noted before, and it is possible to provide a low electricpower consumption device. The surface-emission laser array thus formedof the surface-emission laser diodes of the present invention shows veryhigh electric power transformation efficiency as a whole. Thereby, avery high efficiency device is obtained.

[1221] Further, it becomes possible to conduct parallel opticaltransmission easily by forming a surface-emission laser array, and itbecomes possible to conduct optical transmission and opticaltelecommunication of larger capacity. Further, in the case asurface-emission laser array oscillating in the 1.3 μm band isconstructed by using the active layer material of the surface-emissionlaser diode of the present invention described before, it becomespossible to conduct high speed parallel transmission and communication.Further, in the case a laser array oscillating in the vicinity of the1.5 μm band is formed, it becomes possible to conduct wavelengthmultiplex communication such as WDM, DWDM, and the like, and asurface-emission laser array capable of conducting high speed, largecapacity optical transmission or optical telecommunication is provided.

[1222] [Thirty-Eighth Embodiment]

[1223] A thirty-eighth embodiment of the present invention is asurface-emission laser module that uses the surface-emission laser diodeof the thirty-sixth embodiment or the surface-emission laser array ofthe thirty-seventh embodiment.

[1224]FIG. 151 is a diagram showing an example of the surface-emissionlaser module of the thirty-eighth embodiment. The surface-emission lasermodule of FIG. 151 is constructed by mounting a one dimension monolithicsurface-emission laser array, a micro lens array and a fiber array(silica single mode fiber) on a silicon substrate.

[1225] Here, it should be noted that the surface-emission laser array ofthe thirty-seventh embodiment is provided so as to oppose the opticalfibers and the surface-emission laser array is coupled with the silicasingle mode fibers mounted in the V-grooves that formed on the siliconsubstrate via a micro lens array. The oscillation wavelength of thissurface-emission laser array is in the 1.3 μm band and it is possible toconduct high speed parallel optical transmission by using the silicasingle mode fibers.

[1226] Further, it is possible to obtain a surface-emission laser moduleof low optical absorption loss and low resistance and high electricpower transformation efficiency by using the surface-emission laserarray of the thirty-seventh embodiment as the optical source of thesurface-emission laser module of this thirty-eighth embodiment.

[1227] Thus, the surface-emission laser diode of the thirty-sixthembodiment of reduced optical absorption loss or the surface-emissionlaser array of the thirty-seventh embodiment is used in thesurface-emission laser module of the thirty-eighth embodiment, and as aresult, it becomes possible to provide a surface-emission laser moduleof large slope efficiency, small oscillation threshold current, highelectric power transformation efficiency and low electric powerconsumption.

[1228] In the case of the semiconductor surface-emission laser module inwhich a 1.3 μm band surface-emission laser that uses the GaInNAs mixedcrystal for the active layer material is combined quartz single modefibers, in particular, the 1.3 μm band coincides with the zerodispersion region of quartz, and the construction becomes particularlysuited for the high speed modulation. Thereby, it becomes possible toconduct high speed and large capacity optical telecommunication andoptical transmission by using the surface-emission laser module.

[1229] Further, in the case of the surface-emission laser module thatuses the surface-emission laser diode that oscillates in the 1.5 μmband, it becomes possible to conduct the wavelength multiplexcommunication such as WDM, DWDM, and the like, and high speed and largecapacity optical transmission and optical telecommunication becomespossible. As noted above, it becomes possible to provide asurface-emission laser module of excellent characteristics and enablinghigh speed, large capacity optical transmission and opticaltelecommunication.

[1230] [Thirty-Ninth embodiment]

[1231] A thirty-ninth embodiment of the present invention is an opticalinterconnection system formed of the surface-emission laser diode of thethirty-sixth embodiment, or the surface-emission laser array of thethirty-seventh embodiment, or the surface-emission laser module of thethirty-eighth embodiment.

[1232]FIG. 152 is a diagram showing a parallel optical interconnectionsystem as an example of the thirty-fortieth embodiment. In the opticalinterconnection system of FIG. 152, it should be noted that a devices 1and a devices 2 are connected by using an optical fiber array (quartzsingle mode fiber array). Here, the device 1 at the transmission sideincludes a surface-emission laser module formed of the surface-emissionlaser array of the thirty-thirty-ninth embodiment and a driver circuitthereof. Further, the device 2 at the reception side includes aphotodiode array module and a signal detection circuit.

[1233] In the optical interconnection system of FIG. 152, it is possibleto reduce the optical absorption loss and the resistance and increasethe electric power transformation efficiency by using thesurface-emission laser module of the thirty-eighth embodiment, and it ispossible to obtain an optical transmission system of excellentcharacteristics in terms of electric power consumption. Further, byusing the surface-emission laser module that used the surface-emissionlaser array of the present invention in which GaInNAs is used as theactive layer, it is possible to construct a stable and highly reliableinter connection system with regard to the change of environmentaltemperature.

[1234] While the foregoing description was for the example of theparallel optical interconnection system, it is also possible toconstruct a serial transmission system that uses a single element likethe thirty-second embodiment. Further, it is also possible to use thesurface-emission laser diode of the thirty-sixth embodiment or thesurface-emission laser array of the thirty-seventh embodiment, inaddition to the surface-emission laser module of the thirty-ninthembodiment for the optical interconnection system. Further, the presentinvention can also be applied to the optical interconnection systembetween devices, optical interconnection between boards, between chips,and inside a chip.

[1235] Thus, the thirty-ninth embodiment of the present invention is anoptical interconnection system formed by the surface-emission laserdiode of the thirty-second embodiment, or the surface-emission laserarray of the thirty-seventh embodiment, or the surface-emission lasermodule of the thirty-eighth embodiment. Because of the use of thesurface-emission laser diode or the surface-emission laser array or thesurface-emission laser module of low resistance and low opticalabsorption loss, this optical interconnection system thus constructedhas the feature of high electric power transformation efficiency and lowelectric power consumption.

[1236] Particularly, the optical interconnection system formed of thesurface-emission laser module, in which the 1.3 μm band surface-emissionlaser that uses a GaInNAs mixed crystal semiconductor for the activelayer material is combined with the quartz single mode fiber, is wellsuited to the high speed modulation because of the fact that the 1.3 μmband coincides with the zero dispersion band of quartz, and it becomespossible to conduct high speed and large capacity optical transmission.Further, the surface-emission laser diode that uses the GaInNAs mixedcrystal semiconductor for the active layer can provide highly stableoptical interconnection system of very high reliability in view of thefact that it is possible to obtain the laser oscillation stably up tohigh temperatures even when there is caused a change of environmentaltemperature.

[1237] Thus, according to the thirty-ninth embodiment of the presentinvention, it is possible to provide a highly reliable opticalinterconnection system capable of performing high speed and largecapacity optical transmission and providing high electric powertransformation efficiency, by using the surface-emission laser diode ofthe thirty-sixth embodiment or the surface-emission laser array of thethirty-seventh embodiment or the surface-emission laser module of thethirty-eighth embodiment.

[1238] [Fortieth Embodiment]

[1239] A fortieth embodiment of the present invention is an opticaltelecommunication system formed of the surface-emission laser diode ofthe thirty-first embodiment or the surface-emission laser array of thethirty-seventh embodiment or the surface-emission laser module of thethirty-eighth embodiment.

[1240]FIG. 153 is a diagram showing an optical LAN system as an exampleof the optical telecommunication system of the fortieth embodiment. Theoptical LAN system of FIG. 153 is formed of the surface-emission laserdiode of the thirty-sixth embodiment or the surface-emission laser arrayof the thirty-seventh embodiment or the surface-emission laser module ofthe thirty-eighth embodiment.

[1241] Thus, in the light LAN system of FIG. 153, the surface-emissionlaser diode of the thirty-sixth embodiment, the surface-emission laserarray of the thirty-seventh embodiment, or the surface-emission lasermodule of the thirty-eighth embodiment, is used for the optical sourceof the optical transmission between the server and the core switchand/or between the core switch and the switches and/or between theswitch and each terminal. Thereby, the connection between the devices isachieved by using a quartz single mode fiber or a multiple mode fiber.An example of the physical layer of such an optical LAN may be Giga-bitEthernet of 1000BASE-LX.

[1242] In the optical LAN system of FIG. 153, it becomes possible toreduce the optical absorption loss of the oscillation light and decreasethe device resistance while increasing the electric power transformationefficiency as a result of use of the surface-emission laser diode of thethirty-sixth embodiment or the surface-emission laser array of thethirty-seventh embodiment or the surface-emission laser module of thethirty-eighth embodiment for the optical source of the opticaltransmission, and it becomes possible to provide an optical transmissionsystem of small electric power consumption. In the surface-emissionlaser of the present invention that uses GaInNAs for the active layer,in particular, it is possible to achieve stable oscillation even ifthere is caused change of the environment temperature, drive condition,and the like, and it is possible to construct a highly reliable opticaltelecommunication system.

[1243] Thus, the fortieth embodiment of the present invention is theoptical telecommunication system formed of the surface-emission laserdiode of the thirty-sixth embodiment, or the surface-emission laserarray of the thirty-seventh embodiment, or the surface-emission lasermodule of the thirty-eighth embodiment. Because the use of the lowresistance surface-emission laser diode or surface-emission laser arrayor the surface-emission laser module wherein the optical absorption lossis reduced, it becomes possible to construct an opticaltelecommunication system having high electric power transformationefficiency and low electric power consumption.

[1244] Particularly, the optical telecommunication system that uses thesurface-emission laser module in which the 1.3 μm band surface-emissionlaser having an active layer of the GaInNAs mixed crystal semiconductoris combined with a quartz single mode fiber is suited for high speedmodulation, because of the fact that the 1.3 μm band coincides to thezero dispersion band of quartz. Thereby, it becomes possible to conducthigh-speed and large capacity optical telecommunication and opticaltransmission. Also, the surface-emission laser diode having the activelayer of GaInNAs mixed crystal semiconductor can achieve stableoscillation up to high temperatures even if there is caused change ofthe environment temperatures, and the like, and, it becomes possible toobtain highly reliable optical telecommunication system.

[1245] Thus, by using the surface-emission laser diode of thethirty-sixth embodiment or the surface-emission laser array of thethirty-seventh embodiment or the surface-emission laser module of thethirty-eighth embodiment in the fortieth embodiment, it becomes possibleto conduct high speed and large capacity optical telecommunication, andit becomes possible to provide a highly reliable opticaltelecommunication system having high the electric power transformationefficiency and capable of conducting high-speed and large capacityoptical communication.

[1246] Although the foregoing explanation was made for the case of usingLAN as the optical telecommunication system, the present invention canbe applied also to the other systems such as trunk line system, WAN,MAN, and the like. Further, the terminal can be used in all theinformation device terminals that conduct transfer of information by wayof light.

[1247] [Forty-First Embodiment]

[1248] In a forty-first embodiment, the present invention provides ann-type semiconductor distributed Bragg reflector in which first andsecond semiconductor layers of different refractive indices (bandgap)are stacked, wherein there is provided an intermediate layer(semiconductor layer) between the first and second semiconductor layerswith a refractive index intermediate of the first and secondsemiconductor layers.

[1249] In this forty-first embodiment, depletion of carriers at theheterointerface of the n-type semiconductor distributed Bragg reflectoris suppressed by way of providing the intermediate layer (semiconductorlayer) having an intermediate refractive index (bandgap) of therefractive indices of the first and second semiconductor layers, and theelectrostatic capacitance of the n-type semiconductor distributed Braggreflector is successfully reduced.

[1250] In a semiconductor heterointerface of a semiconductor distributedBragg reflector formed by semiconductor layers of different refractiveindices (bandgap), there is caused a potential distribution called spikeor notch at the heterointerface as a result of the difference of dopingdensity and electron affinity between the semiconductors. In the partformed with spike, for example, there is formed a high potential barrierfor the carriers, and depletion of the carriers is caused as a result ofthis. In the part where a notch is formed, on the other hand, there iscaused accumulation of carriers because of the decrease of the potentialbarrier. Thus, the distribution of carriers in the heterointerface isnot uniform, and the device characteristic is influenced heavily by theexistence of the heterointerface.

[1251] It is known in the case of a p-type semiconductor distributedBragg reflector that the device resistance increases significantly dueto the influence of the spike at the heterointerface in view of the factthat the holes constituting the majority carrier (hole) have a largeeffective mass. In the case of an n-type semiconductor distributed Braggreflector, the influence of the heterointerface on the device resistanceis thought as being smaller, in view of the fact that the effective massof the electrons constituting the majority carrier is smaller ascompared with holes. However, no detailed examination has been madeconventionally about the influence of the heterointerface for the caseof an n-type semiconductor distributed Bragg reflector. However, thephenomenon of depletion of carriers is caused irrespective of theconductivity type in the heterointerface as noted above, andelectrostatic capacitance is caused inevitably as a result of thecarrier depletion.

[1252] Hereinafter, explanation will be made about the current-voltagecharacteristic of FIG. 167 based on the assumption that theheterointerface provides the influence to the device characteristic. Itshould be noted that the current-voltage characteristic of FIG. 167compares the measurement result conducted on the n-type semiconductordistributed Bragg reflector formed of AlAs/GaAs shown FIG. 146 for thecase of pulse measurement and for the case of CW (continuation wave)measurement. As for the device of FIG. 168, it should be noted that thedevice is an n-type semiconductor distributed Bragg reflectors formed onan n-GaAs substrate by forming 35 stacks of AlAs/GaAs unit structure byconducting a crystal growth process. In the device of FIG. 168, ohmicelectrodes are provided to the rear surface of the n-GaAs substrate andfurther to the removal part (stripe-formed oxide removal part having awidth of 40 μm and a length of 500 μm) formed in the SiO₂ insulationlayer on the n-type semiconductor distributed Bragg reflector layer. Inthe n-type semiconductor distributed Bragg reflector of FIG. 168, itshould be noted that the intermediate layer (compositional graded layer)is not provided to the interface of the AlAs/GaAs structure, and thus,the composition changes steeply at each interface, In FIG. 167, it canbe seen that there exists a large difference in the current-voltagecharacteristic between the CW measurement and the pulse measurement. Inthe pulse measurement, in particular, it can be seen that theapplication voltage is high and the device resistance is large. Further,in the case of the CW measurement, too, it can be seen that negativeresistance appears in the vicinity of the current value of 500 mA. Fromthis, it can be seen the situation in which the depletion oraccumulation of carriers at the heterointerface exerting influence onthe device characteristics. Thus, there is caused non-linearity(negative resistance) in the current-voltage characteristic for then-type distributed Bragg reflector having a steep heterointerface,wherein the cause of this non-linearity is attributed to tunneling ofcarriers, and the like, at the hetero barrier. Further, there is causedchange of current-voltage characteristic due to the difference of themeasurement condition of the device.

[1253]FIG. 169, on the other hand, is a diagram showing thecurrent-voltage characteristic of the device having a structure similarto that FIG. 146 for the case there is provided a linear compositionalgraded layer at each interface of the AlAs/GaAs structure constitutingthe n-type semiconductor distributed Bragg reflector as an intermediatelayer. In FIG. 169, the cases in which the thickness of the linear shapecompositional graded layer is set to 20 nm and 30 nm are shown In FIG.169, it should be noted that the difference between the pulsemeasurement and the CW measurement is reduced as compared with the caseof FIG. 167, and the linearity of the characteristics is improved. Thus,the difference of the current-voltage characteristic between themeasurement conditions (drive condition) is improved substantially, andthe non-linearity (negative resistance) observed at the time of the CWmeasurement is also improved substantially. Thus, the influence of theheterointerface on the current-voltage characteristics is reducedsubstantially, and along with this, the electrostatic capacitance causedby the depletion at the interface is reduced substantially.

[1254] In a surface-emission laser diode, the device resistance and theelectrostatic capacitance determine the upper limit of modulation speedof the device by way of miniaturization of the device size and oxidationconfinement diameter. Accordingly, it is very important to reduce thedevice resistance and capacitance as small as possible in order torealize high speed modulation exceeding 10 Gbps. Thus, it is importantto reduce the capacitance at the interface of the semiconductordistributed Bragg reflector in addition to providing the oxidationconfinement layer Further, the observed change of the current-voltagecharacteristic changes with the measurement condition as shown in FIG.167 and the non-linearity in the current-voltage characteristic are notdesirable in view point of practical use of the device.

[1255] In the case of providing an intermediate layer (semiconductorlayer (a linear compositional graded layer in the present case)) havinga refractive index (bandgap) intermediate of the semiconductor layersconstituting an n-type semiconductor distributed Bragg reflector as inthe case of the forty-first embodiment, it is possible to suppress theoccurrence of the potential distribution such as spike, notch, and thelike, as it shown in FIG. 169. By smoothing the potential barrier at theheterointerface like this, it becomes possible to drastically reduce theaccumulation or depletion of carriers taking place at theheterointerface conventionally. With this, the electrical resistance isalso reduced. Further, by reducing the influence of the heterointerfaceeffectively, it is also possible to reduce the non-linearity of thecurrent-voltage characteristic of a n-type distributed Bragg reflectorand the change of the current-voltage characteristic by drive condition.Further, as a result of suppressing of the depletion of carriers at theheterointerface, the electrostatic capacitance by the depletion regionis reduced substantially. Like this, it becomes possible to improve thecurrent-voltage characteristic of the n-type semiconductor distributedBragg reflector. Further, as a result of decrease of the electrostaticcapacitance, driving is easily made in a surface-emission laser diodethat uses an n-type semiconductor distributed Bragg reflector for thereflector as compared with a conventional device, and further high-speedmodulation becomes possible.

[1256] Thus, in the forty-first embodiment, there is provided anintermediate layer (semiconductor layer) between first and secondsemiconductor layers having respective, mutually different refractiveindices and constituting an n-type semiconductor distributed Braggreflector, such that the intermediate layer has a refractive indexintermediate of the first and second semiconductor layers so as todecrease low the influence of the electrostatic capacitance of then-type semiconductor distributed Bragg reflector. By smoothing thepotential barrier at the heterointerface, it becomes possible to reducethe change of the current-voltage characteristic caused by thedifference of drive (measurement) condition and the non-linearity of thecurrent-voltage characteristics. Further, the electrostatic capacitancecaused by the depletion of carriers is also reduced. From above, thepresent embodiment can provide an n-type distributed Bragg reflectorhaving a reduced electrostatic capacitance at the semiconductorheterointerface.

[1257] [Forty-Second Embodiment]

[1258] A forty-second embodiment of the present invention has aconstruction in which the thickness of the above-mentioned intermediatelayer is set larger than 20 nm] in the distributed Bragg reflector ofthe forty-first embodiment.

[1259] In the forty-second embodiment, it becomes possible to obtain ann-type distributed Bragg reflector having excellent electriccharacteristics such as reduced non-linearity in the current-voltagecharacteristics, reduced change of current-voltage characteristics bymeasurement condition and reduced electrostatic capacitance at theheterointerface, by setting the thickness of the intermediate layer tothe foregoing thickness and by smoothing the potential barrier at theheterointerface.

[1260] Referring to FIG. 169 again, it can be seen that the differencebetween the CW measurement and the pulse measurement is reducedsubstantially in the structure of FIG. 169 in which there is providedthe compositional graded layer (intermediate layer) with the thicknessof 20 nm, as compared with the structure of FIG. 167 in which theintermediate layer is not provided. By providing the intermediate layerto each of the interfaces of the n-type distributed Bragg reflector withthe thickness of 20 nm or more, these problems are reducedsignificantly. Thus, the difference of the current-voltagecharacteristic between the pulse measurement and the CW measurement isreduced substantially by providing the compositional graded layer withthe thickness of 20 nm. Further, it can be seen that the non-linearityof the current-voltage characteristic seen in the CW measurement iseliminated. These results show that the potential barrier at theheterointerface of the Bragg reflector can be smoothed substantially byproviding the compositional graded layer of the thickness of 20 nm.Along with this, the electrostatic capacitance caused by depletion ofthe carriers at the heterointerface can be reduced substantially.

[1261] In the forty-second embodiment, the influence of theheterointerface is reduced substantially and the current-voltagecharacteristic of the n-type distributed Bragg reflector is improvedsubstantially by providing a compositional graded layer with a thicknesslarger than 20 nm, and the electrostatic capacitance at theheterointerface is reduced.

[1262] [Forty-Third Embodiment]

[1263] A forty-third embodiment of the present invention has aconstruction in which the thickness of the above-mentioned intermediatelayer set to 30 nm or more in the n-type distributed Bragg reflector ofthe forty-first embodiment.

[1264] In the forty-third embodiment, it is possible to obtain an n-typedistributed Bragg reflector of excellent electric characteristics inwhich the influence of the heterointerface is more effectively reducedand the electrostatic capacitance at the heterointerface is moreeffectively reduced, by setting the thickness of the intermediate layerto the above thickness.

[1265] The reason that the influence of the heterointerface is reducedis that the heterointerface is smoothed by the intermediate layer andthe accumulation or depletion of the carriers at the interface issuppressed. This function is enhanced with increasing thickness of theintermediate layer. Here, reference is made again to the current-voltagecharacteristic of FIG. 169. In FIG. 169, it can be seen that thedifferences of current-voltage characteristics between the pulsemeasurement and the CW measurement is reduced and the non-linearity atthe time of the CW measurement are reduced as a result of the effect ofsmoothing of the potential barrier at the heterointerface caused by theintermediate layer, for both of the specimens in which the thickness ofthe compositional graded layer (intermediate layer) is changed.Especially, it can be seen that the aforementioned point is improvedmore in the specimen in which the compositional graded layer having thethickness of 30 nm is provided In this case, the difference between theCW measurement and the pulse measurement is almost vanished. Therefore,it is possible to enhance the improving effect by setting the thicknessof the intermediate layer to be 30 nm or more.

[1266] In the forty-third embodiment, the influence of theheterointerface is reduced more effectively as noted before by providingthe compositional graded layer of the thickness of 30 nm or more. Thus,the current-voltage characteristic of the n-type distributed Braggreflector is improved more effectively and the electrostatic capacitanceat the heterointerface is reduced effectively. As noted above, it ispossible to reduce the change of the current-voltage characteristics andnon-linearity of the current-voltage characteristics effectively in thepresent embodiment, and the electrostatic capacitance at theheterointerface can be reduced more effectively. From above, an n-typedistributed Bragg reflector having excellent electric characteristics isobtained.

[1267] [Forty-Fourth Embodiment]

[1268] A forty-fourth embodiment of the present invention has aconstruction in which the thickness t [nm] of the above-mentionedintermediate layer is set with respect to the reflection wavelength λ[um] of the distributed Bragg reflector to fall in the range of20<t≦(50λ−15) [nm] in the n-type distributed Bragg reflector of theforty-first embodiment.

[1269] When the thickness of the intermediate layer is excessive, thereflectivity of the distributed Bragg reflector falls off rapidly. Onthe other hand, the effect of planarizing the heterointerface isenhanced when the thickness of the intermediate layer is increased.Thus, there exists an optimum range for the intermediate layer thicknessthat satisfy both of these requirements. In the present embodiment, ann-type distributed Bragg reflector having excellent electriccharacteristics is obtained by reducing the effect of theheterointerface while simultaneously maintaining the high reflectivityby choosing the thickness of the intermediate layer with respect to thereflection wavelength λ[μm] of the distributed Bragg reflector so as tofall in the range of 20<t≦(50λ−15) [nm].

[1270] In the forty-fourth embodiment, it becomes possible to obtain ann-type distributed Bragg reflector of excellent electricalcharacteristics in which the influence of the heterointerface is reducedwhile maintaining high reflectivity, by making use of the aforementionedconstruction. Larger the refractive-index difference between thesemiconductor layers constituting the distributed Bragg reflector, andsteeper the interface, the reflectivity of the distributed Braggreflector increases. Thus, with increasing thickness of thecompositional graded layer, the reflectivity tends to decreasegradually.

[1271]FIG. 170 shows this situation. Thus, FIG. 170 is a diagram showingthe differential coefficient of the reflectivity change to the thicknessof the compositional graded layer for the case of the distributed Braggreflector formed of AlAs/GaAs for each wavelength band. It should benoted that the stacking number of the distributed Bragg reflector is setsuch that the reflectivity has just exceeded 99.9% in each of thewavelength bands. From FIG. 170, it can be seen that the reflectivitystarts to change (decline) gradually at first while the reflectivitychanges sharply from a certain value. In order to facilitateunderstanding of this situation, tangent lines are drawn to thethickness in which the differential coefficient starts to change in FIG.170 at several wavelength bands. From these tangent lines, it can beseen that the differential coefficient starts to change sharply in thevicinity where differential coefficient becomes 0.01 in any of thewavelength bands. Thus, the thickness corresponding to the differentialcoefficient of 0.01 is the thickness in which the reflectivity of thedistributed Bragg reflector starts to increase sharply (thresholdthickness). When the thickness of the compositional graded layer is setlarger than this threshold thickness, the reflectivity of thedistributed Bragg reflector falls off sharply. The thickness of thecompositional graded layer (threshold thickness) providing thedifferential coefficient of 0.01 can be read from the next table (Table13) for the respective wavelength bands as follows. There exists alinear relationship as follows (equation 1) between the reflectionwavelength λ [um] of the Bragg reflector and the threshold thickness.TABLE 13 Reflection 1.1 μm 1.3 μm 1.3 μm 1.7 μm wavelength Threshold  40nm  50 nm  60 nm  70 nm thickness

[1272] Here, the reflection wavelength is the wavelength in which thereflectivity becomes highest in the reflection band of the distributedBragg reflector. Further, the thickness of the layer constituting theBragg reflector and having the refractive index n is represented withrespect to the reflection wavelength λ as λ/4n in the structure in whichthe intermediate layer is not provided.

[1273] As noted above, the effect of improving the difference of thecurrent-voltage characteristics by the drive condition, thenon-linearity of the current-voltage characteristics, and theaccumulation or depletion of the carriers at the heterointerface becomesconspicuous in the case the thickness of the intermediate layer islarge. From this viewpoint, it is desirable to provide a thickintermediate layer. However, there arise problems as noted above in thecase that the thickness of the intermediate layer is too large. Thus,there is an optimal range for the thickness of the intermediate layer.

[1274] From the aforementioned result, the optimum thickness range ofthe intermediate layer can be represented with respect to the reflectionwavelength λ [μm] of the distributed Bragg reflector as 20<t≦(50λ−15)[nm], in view of the electric characteristics (current-voltagecharacteristics) and in view of the optical characteristics(reflectivity). Here, the inequality that determines the range of theintermediate layer thickness t holds within the range of λ>0.7 μm, andcan be applied to the n-type distributed Bragg reflector having thereflection wavelength longer than this wavelength. For example, thedevice that uses the AlGaAs system material to the active layer canobtain laser oscillation in typically in the 0.78 μm-0.85 μm band. Inthe case a GaInAs system is used, it is possible obtain the laseroscillation in the 0.98 μm band to 1.2 μm band. In the case of using theGaInNAsSbP material system, it is possible to obtain laser oscillationin the wavelength region longer than the 1.2 μm band.

[1275] Thus, the n-type distributed Bragg reflector having theintermediate layer with the thickness falling in the range determined bythe forty-fourth embodiment can be used for a laser based on such amaterial and having the wavelength longer than 0.7 μm or a device suchas optical modulator.

[1276] [Forty-Fifth Embodiment]

[1277] A forty-fifth embodiment of the present invention provides asurface-emission laser diode that uses the n-type semiconductordistributed Bragg reflectors of any of forty-first through forty-fourthembodiments.

[1278] In the surface-emission laser diode of the forty-fifthembodiment, the electrostatic capacitance of the surface-emission laserdiode is reduced and high speed modulation becomes possible as a resultof use of the n-type semiconductor distributed Bragg reflectors of anyof the forty-first through forty-fourth embodiments.

[1279] Thus, in the surface-emission laser diode of the forty-fifthembodiment, the potential distribution such as spike or notch formed atthe interface of the n-type semiconductor distributed Bragg reflector issmoothed by using the n-type semiconductor distributed Bragg reflectorsof any of the forty-first forty-through forty-fourth embodiments. Thus,the influence of depletion or accumulation of carriers caused by thespike or notch is successfully reduced as compared with a conventionalsurface-emission laser diode, and surface-emission laser diode of theforty-fifth embodiment provides reduced electrical resistance as well asreduced electrostatic capacitance at the heterointerface.

[1280] Thus, the surface-emission laser diode of the forty-fifthembodiment has a structure suitable for high speed modulation ascompared with a conventional surface-emission laser diode. As a resultof decrease of the electrostatic capacitance of the n-type semiconductordistributed Bragg reflector, it can easily perform high speed modulationexceeds 10 Gbps.

[1281] [Forty-Sixth Embodiment]

[1282] A forty-sixth embodiment of the present invention provides asurface-emission laser diode in which an n-type semiconductordistributed Bragg reflector and a p-type semiconductor distributed Braggreflector are provided across an active layer, wherein the n-typesemiconductor distributed Bragg reflector is processed to form a mesa.

[1283] Thus, in this forty-sixth embodiment, the n-type semiconductordistributed Bragg reflector is processed to form a mesa in thesurface-emission laser diode in which the n-type semiconductordistributed Bragg reflector and the p-type semiconductor distributedBragg reflector are provided across the active layer, and as a result,the path of the current injected to the device part (cavity region) ofthe laser diode through the n-type semiconductor distributed Braggreflector is limited. As a result, the area of the n-type semiconductordistributed Bragg reflector contributing to the electrostaticcapacitance is decreased, and the electrostatic capacitance is reduced,and as a result, high speed modulation becomes possible.

[1284] Thus, in the forty-sixth embodiment, the n-type semiconductordistributed Bragg reflector is processed to form a mesa in thesurface-emission laser diode in which the n-type semiconductordistributed Bragg reflector and the p-type semiconductor distributedBragg reflector are provided across the active layer (More specifically,the n-type semiconductor distributed Bragg reflector is removed by anetching process except for the region forming the device part (cavityregion) such that the etching reaches the substrate surface as will beexplained later with reference to FIG. 157). As explained before, thereis formed a potential distribution of spike or notch at theheterointerface formed two semiconductor layers of different electronaffinity, and there is formed electrostatic capacitance as a result ofdepletion of carriers. Further, it has been practiced in a conventionalsurface-emission laser element to provide an oxidation confinementstructure for reducing the threshold current by oxidizing thesemiconductor layer that contains Al such as AlAs, and the like. Becauseholes have a shorter diffusion length as compared with electrons andthus higher confinement efficiency is obtained with holes as comparedwith electrons, the foregoing confinement structure is usually providedin the p-type semiconductor distributed Bragg reflector. Generally, noconfinement structure is provided in the n-type semiconductordistributed Bragg reflector, and injection of electrons is conducted asthe majority carrier from the electrode formed on the entire rearsurface of the substitute substrate. Thus, conventional surface-emissionlaser diode generally has a wider current path for the electrons ascompared with the mesa diameter that constitutes the path of the holes,and the area for the electron current in the n-type semiconductordistributed Bragg reflector pass is larger than the p-type semiconductordistributed Bragg reflector. As electrostatic capacitance increases inproportion to the area, the electrostatic capacitance of the n-typesemiconductor distributed Bragg reflector increases in correspondence tothe spreading of the current. This has been the cause of increase of thedevice capacity.

[1285] In this forty-sixth embodiment, the current path of the electronsis confined by removing the n-type semiconductor distributed Braggreflector by way of an etching process except for a region correspondingto the device part (cavity region) to the region left at the time of theforegoing etching removal process. With this, it becomes possible toreduce the electrostatic capacitance caused by the n-type semiconductordistributed Bragg reflector. Thus, the surface-emission laser diode ofthe forty-sixth embodiment is a structure suitable for high speedmodulation as compared with conventional surface-emission laser diodes.In the surface-emission laser diode of the forty-sixth embodiment, itbecomes possible to perform high speed modulation of 10 Gbps or moreeasily as a result of reduction of the electrostatic capacitance of then-type semiconductor distributed Bragg reflector.

[1286] By confining the current path in the n-type semiconductordistributed Bragg reflector to the degree comparable with the mesadiameter of the device part (cavity region) in the surface-emissionlaser diode in the forty-sixth embodiment, the area of the n-typesemiconductor distributed Bragg reflector that contributes to theelectrostatic capacitance is reduced and the electrostatic capacitanceof the device is reduced. Thereby, high speed modulation becomespossible.

[1287] [Forty-Seventh Embodiment]

[1288] A forty-seventh embodiment of the present invention provides asurface-emission laser diode in which an n-typesemiconductor-distributed Bragg reflector and a p-type semiconductordistributed Bragg reflector are provided across an active layer whereinthe region other than the cavity region of the n-type semiconductordistributed Bragg reflector is formed to have a high resistance.

[1289] In the surface-emission laser diode of the forty-seventhembodiment in which the n-type semiconductor distributed Bragg reflectorand the p-type semiconductor distributed Bragg reflector are providedacross the active layer, the resistance of the n-type semiconductordistributed Bragg reflector is increased in the region other than thecavity region, and the path of the current injected to the device region(cavity region) through the n-type semiconductor distributed Braggreflector is confined. Thereby, the area of the n-type semiconductordistributed Bragg reflector, which contributes to the electrostaticcapacitance, is decreased, and the electrostatic capacitance iseffectively reduced. Thus, high speed modulation becomes possible.

[1290] Thus, in the forty-seventh embodiment, the resistance of then-type semiconductor distributed Bragg reflector is increased except forthe region of the device part (cavity region). As a result ofconfinement of the current path of the electrons injected from thesubstrate side to only a part of the mesa region of the n-typesemiconductor distributed Bragg reflector, the area of the n-typesemiconductor distributed Bragg reflector contributing to theelectrostatic capacitance is reduced similarly to the case of theforty-sixth embodiment. Thus, it becomes possible to reduce theelectrostatic capacitance of the surface-emission laser diode ascompared with the case of a conventional surface-emission laser diode,and a structure suitable for the high speed modulation is obtained.

[1291] In other words, the area of the n-type semiconductor distributedBragg reflector contributing to the electrostatic capacitance is reducedby confining the current path in the n-type semiconductor distributedBragg reflector to the mesa diameter of the device part (cavity region)as in the case of the forty-sixth embodiment also in thesurface-emission laser diode of the forty-seventh embodiment, andelectrostatic capacitance of the device is effectively reduced. Thereby,high speed modulation becomes possible.

[1292] For the method of increasing the resistance of an n-typesemiconductor distributed Bragg reflector, it is possible to usehydrogen ion implantation, and the like, as will be described later withreference to forty-fifth embodiment. In view of the fact that thedistance between the active region and the etching surface is small, itis easy to achieve various advantages such as excellent heat radiation,easiness of burying, excellent physics strength, and the like, as aresult of use of the hydrogen ion implantation process. Particularly,the feature of excellent heat radiation enables high output operation ofthe laser diode. In fact, it becomes possible to perform high speedmodulation of 10 Gbps or more easily in the surface-emission laser diodeof the forty-seventh embodiment by reducing the electrostaticcapacitance of the n-type semiconductor distributed Bragg reflector.

[1293] [Forty-Eighth Embodiment]

[1294] A forty-eighth embodiment of the present invention provides asurface-emission laser diode of the forty-sixth or forty-seventhembodiment having the above-mentioned n-type semiconductor distributedBragg reflector in which first and second semiconductor layers ofdifferent refractive indices are stacked, wherein there is provided anintermediate layer (semiconductor layer) having a refractive indexintermediate of the refractive indices of the first and secondsemiconductor layers.

[1295] In the forty-eighth embodiment, there is provided an intermediatelayer (semiconductor layer) in the surface-emission laser diode of theforty-sixth or forty-seventh embodiment in the n-type semiconductordistributed Bragg reflector between the first and second semiconductorlayers having respective, mutually different refractive indices, with arefractive index intermediate of the foregoing first and secondsemiconductor layers with this, the electrostatic capacitance of theheterointerface in the n-type semiconductor distributed Bragg reflectorcan be reduced significantly. Thereby, further high speed modulationbecomes possible.

[1296] Thus, in the forty-eighth embodiment, there is provided anintermediate layer (semiconductor layer) to the heterointerface formedby the first and second semiconductor layers constituting the n-typesemiconductor distributed Bragg reflector in the surface-emission laserdiode of the forty-sixth or forty-seventh embodiment with a refractiveindex (bandgap) intermediate of the two semiconductor layers, andbecause of this, the potential at the heterointerface is smoothed as aresult of providing the intermediate layer (semiconductor layer) to theheterointerfaces of the first and second semiconductor layers asexplained in any of forty-first through forty-fifth embodiments, anddepletion or accumulation of carriers is reduced. With this, theelectrostatic capacitance caused by the depletion is reducedsignificantly. By confining the path of the electrons of the n-typesemiconductor distributed Bragg reflector to the mesa region similarlyto the forty-sixth or forty-seventh embodiment. Thus, by combining theseas in the case of the forty-eighth embodiment, the area of thesemiconductor distributed Bragg reflector that contributes effectivelyto the capacitance is reduced, the electrostatic capacitance of thesurface-emission laser diode is reduced further, and it becomes possibleto obtain the structure very suitable for high speed modulation In fact,the surface-emission laser diode of the forty-eighth embodiment canperform high speed modulation of 10 Gbps or more easily as a result ofdecrease of the electrostatic capacitance of the n-type semiconductordistributed Bragg reflector as noted above.

[1297] [Forty-Ninth Embodiment]

[1298] A forty-ninth embodiment of the present invention, provides asurface-emission laser diode of the forty-sixth or forty-seventhembodiment in which the n-type semiconductor distributed Bragg reflectorincludes an intermediate layer between the first and secondsemiconductor layers having respective, mutually different refractiveindices with a refractive index intermediate of the first and secondsemiconductor layers and wherein the thickness of the above-mentionedintermediate layer is set to be larger than 20 [nm].

[1299] By using the construction of the forty-ninth embodiment, itbecomes possible to reduce the area of the n-type semiconductordistributed Bragg reflector contributing to the electrostaticcapacitance by confining the path of the electrons current, and theelectrostatic capacitance of the heterointerface of the n-typesemiconductor distributed Bragg reflector is reduced. Thereby, highspeed modulation becomes possible.

[1300] Thus, by providing the semiconductor layer explained before tothe heterointerface as explained with reference to forty-first orforty-second embodiment, the depletion or accumulation of the carriersis reduced as a result of smoothing of the potential distribution at theheterointerface as compared with the case in which such an interfacelayer is not provided, and the electrostatic capacitance caused by thedepletion is reduced significantly. Further, by confining the path ofthe electrons by etching of the n-type semiconductor distributed Braggreflector or increase of resistance as set forth in forty sixth or fortyseventh embodiment, the area of the semiconductor distributed Braggreflector contributing to the capacitance can be reduced. Thus, bycombining these features as set forth in the present embodiment, it ispossible to reduce the electrostatic capacitance of the device further,and a structure extremely suitable for high speed modulation isobtained. The device of the present embodiment can perform the highspeed modulation of 10 GPs or more as a result of decrease of theelectrostatic capacitance of the n-type distributed Bragg reflector.

[1301] [Fiftieth Embodiment]

[1302] A fiftieth embodiment of the present invention has a constructionin which the n-type semiconductor distributed Bragg reflector of thesurface-emission laser diode of the forty-sixth or forty-seventhembodiment includes an intermediate layer between the first and secondsemiconductor layers having respective, mutually different refractiveindices with a refractive index intermediate of the first and secondsemiconductor layers, wherein the thickness of the intermediate layer isset to be 30 [nm] or more.

[1303] By using the construction of the fiftieth embodiment, it becomespossible to reduce the area of the n-type semiconductor distributedBragg reflector contributing to the electrostatic capacitance byconfining the path of the electron current, and it becomes possible toreduce the electrostatic capacitance of the heterointerface of then-type semiconductor distributed Bragg reflector, Thereby, high speedmodulation becomes possible.

[1304] Thus, by providing the semiconductor layer explained before tothe heterointerface as explained with reference to forty-first orforty-second embodiment, the depletion or accumulation of the carriersis reduced sufficiently as a result of smoothing of the potentialdistribution at the heterointerface, and the electrostatic capacitancecaused by the depletion is reduced furthermore. Further, by confiningthe path of the electrons by etching of the n-type semiconductordistributed Bragg reflector or increase of resistance as set forth inforty sixth or forty seventh embodiment, the area of the semiconductordistributed Bragg reflector contributing to the capacitance can bereduced. Thus, by combining these features as set forth in the presentembodiment, it is possible to reduce the electrostatic capacitance ofthe device furthermore effectively, and a structure extremely suitablefor high speed modulation is obtained. The device of the presentembodiment can perform the high speed modulation of 10 GPs or more as aresult of decrease of the electrostatic capacitance of the n-typedistributed Bragg reflector.

[1305] [Fifty-First Embodiment]

[1306] A fifty-first embodiment of the present invention has aconstruction in which the n-type semiconductor distributed Braggreflector forming the surface-emission laser diode of the forty-sixth orforty-seventh embodiment includes an intermediate layer between thefirst and second semiconductor layers having respective, mutuallydifferent refractive index with a refractive intermediate of the firstand second semiconductor layers, wherein the thickness t [nm] of theabove-mentioned intermediate layer is determined in relation to thereflection wavelength λ [um] of the distributed Bragg reflector so as tofall in ranges of 20<t≦(50λ−15) [nm].

[1307] By using the construction of the fifty-first embodiment, itbecomes possible to reduce the area of the n-type semiconductordistributed Bragg reflector that contributing to the electrostaticcapacitance by limiting the path of the electron current, and theelectrostatic capacitance of the heterointerface of a n-typesemiconductor distributed Bragg reflector is reduced. Thereby, highspeed modulation becomes possible.

[1308] Thus, the potential distribution at the heterointerface issmoothed by providing the semiconductor layer to the heterointerfaces asexplained with reference to the forty-first through forty-thirdembodiments, and depletion or accumulation of carriers is reduced.Thereby, the electrostatic capacitance caused by the depletion issignificantly reduced. Further, by choosing the thickness of theintermediate layer to the foregoing range, high reflectivity ismaintained for the n-type distributed Bragg reflector. Thus, asurface-emission laser diode characterized by low threshold current isobtained.

[1309] By confining the path of the electrons to the mesa region byetching or increase of resistance of the n-type semiconductordistributed Bragg reflector, the area of the semiconductor distributedBragg reflector contributing to the capacitance is reduced. Thus, bycombining these features as in the present embodiment, it is possible toreduce the electrostatic capacitance further, and a structure extremelysuitable for high speed modulation is obtained. The device of thepresent embodiment can perform the high speed modulation of 10 Gbps ormore as a result of decrease of the electrostatic capacitance of then-type semiconductor Bragg reflector.

[1310] [Fifty-Second Embodiment]

[1311] A fifty-second embodiment of the present invention has aconstruction of the surface-emission laser diode of any the forty-fifththrough fifty-first embodiments, wherein the present embodiment has afeature in that the active layer is formed of a group III element and agroup V element, the group III element of the active layer being any orall of Ga and In, the group V element of the active layer being any orall of As, N, Sb and P.

[1312] In this fifty-second embodiment, the active layer is formed ofthe group III element and the group V element in the surface-emissionlaser diode of any of the forty-fifth through fifty-first embodiments,wherein the group III element of the active layer is any or all of Gaand In, while the group V element of the active layer is any or all ofAs, N, Sb and P. With this, it becomes possible to obtain asurface-emission laser diode oscillating at wavelength longer than 1.1μm including the 1.3 μm band and 1.5 μm band, which are important foroptical-fiber telecommunication, on a GaAs substrate. Further, becauseit is possible to use GaAs for the semiconductor substrate, a highquality semiconductor distributed Bragg reflector formed of AlGaAs/GaAsfor the cavity reflector. Thereby, it becomes possible to provide asurface-emission laser diode of excellent characteristics and lowthreshold for the use in long wavelength band application.

[1313] Thus, in the fifty-second embodiment, the group III element ofthe active layer is chosen from any or all of Ga and In and the group Velement of the active layer is chosen from any or all of As, N, Sb andP, while it should be noted that the active layer formed of thesematerials can be grown on a GaAs substrate by way of crystal growthprocess. Thus, it is possible to obtain a surface-emission laser diodehaving excellent characteristics in terms of reflectivity, thermalconductivity, process controllability (crystal growth or selectiveoxidation of the Al(Ga)As mixed crystal) by using the AlGaAs systemmaterial for the DBR. Further, it is possible to obtain the oscillationat the wavelength of 0.85 μm band and 0.98 μm band and further at thewavelength longer than 1.1 μm, including the 1.3 μm band and 1.5 μm bandwhich are important for optical-fiber telecommunication, by using thesematerials for the active layer.

[1314] Thereby, it becomes possible to conduct high speed opticaltelecommunication by combining the laser device of the 1.3 μm wavelengthband with a silica single mode laser. Further, it is possible to conductlarge capacity communication by using DWDM by the device of the 1.5 μmband.

[1315] Among the materials (active layer material) mentioned above, itshould be noted that the mixed crystal of GaInN(Sb)As can achieve theoscillation at the wavelength longer than 1.1 μm. Further, with thelayer of the foregoing GaInN(Sb)As material, the band discontinuity atthe conduction band is increased with respect to the GaAs layer used fora carriers confinement layer, and because of this, the overflow of theelectron is successfully reduced. As a result, it is possible to obtaina stable oscillation even at high temperatures. In addition to these,the electrostatic capacitance of the n-type semiconductor distributedBragg reflector is reduced in the surface-emission laser diode of thepresent invention. Because of this, it becomes possible to perform highspeed modulation more easily as compared with a conventional device.According to the present invention, it becomes possible to provide asurface-emission laser diode suitable for optical telecommunication andoptical transmission.

[1316] [Fifty-Third Embodiment]

[1317] A fifty-third embodiment of the present invention is asurface-emission laser array formed of the surface-emission laser diodeof any of the forty-fifth through fifty-second embodiments.

[1318] As the surface-emission laser array of the fifty-third embodimentis formed of the surface-emission laser diode of any of the forty-fifththrough fifty-second embodiments, the electrostatic capacitance of thedevice is small and high speed modulation becomes possible.

[1319] Thus, in the surface-emission laser diode constituting thesurface-emission laser array of the fifty-third embodiment, theelectrostatic capacitance caused by the n-type semiconductor distributedBragg reflector is reduced, and with this, high speed modulation becomespossible. Thus, high speed modulation becomes possible similarly in thesurface-emission laser array formed of such surface-emission laserdiodes. Further, by constructing a surface-emission laser array,parallel optical transmission is easily conducted, and it becomespossible to conduct high speed and large capacity optical transmissionand optical telecommunication. Thus, in the fifty-third embodiment, asurface-emission laser array capable of performing high speed and largecapacity optical transmission and optical telecommunication is provided.

[1320] [Fifty-Fourth Embodiment]

[1321] A fifty-fourth embodiment of the present invention is asurface-emission laser module formed of the surface-emission laserdiodes of any of the forty-fifth through fifty-second embodiments or thesurface-emission laser array of the fifty-third embodiment.

[1322] As the surface-emission laser module of the fifty-fourthembodiment is formed of the surface-emission laser diode of any of theforty-fifth through fifty-second embodiments or the surface-emissionlaser array of the fifty-third embodiment, it becomes possible toconduct high speed and large capacity optical transmission and opticaltelecommunication.

[1323] Thus, a surface-emission laser diode or a surface-emission laserarray in which the electrostatic capacitance is reduced in the n-typesemiconductor distributed Bragg reflector and capable of performinghigher speed modulation, is used in the surface-emission laser module ofthe fifty-fourth embodiment, and because of this, high speed and largecapacity optical transmission and optical telecommunication becomepossible More specifically, it should be noted that the 1.3 μm bandcoincides to the zero dispersion band of quartz in the case of thesurface-emission laser module in which a 1.3 μm band surface-emissionlaser diode that uses the GaInNAs mixed crystal for the active layer iscombined with a quartz single mode fiber as in the case of the example 7to be described later. Because of this, such a construction is wellsuited to high speed modulation. By using this surface-emission lasermodule, it is possible to conduct high speed and large capacity opticaltelecommunication and optical transmission. Thus, the fifty-fourthembodiment provides a surface-emission laser module capable ofconducting high speed and large capacity optical transmission andoptical telecommunication.

[1324] [Fifty-Fifth Embodiment]

[1325] A fifty-fifth embodiment of the present invention is an opticalinterconnection system formed of the surface-emission laser diode of anyof the forty-fifth through fifty-second embodiments or asurface-emission laser array of the fifty-third embodiment or thesurface-emission laser module of the fifty-fourth embodiment.

[1326] As the optical interconnection system of the fifty-fifthembodiment is formed of the surface-emission laser diode of any of theforty-fifth through fifty-second embodiments or the surface-emissionlaser array of the fifty-third embodiment or the surface-emission lasermodule of the fifty-fourth embodiment, it becomes possible to conducthigh speed and large capacity optical transmission.

[1327] Thus, a surface-emission laser diode or a surface-emission laserarray or a surface-emission laser module in which the electrostaticcapacitance is reduced in the n-type semiconductor distributed Braggreflector and capable of performing higher speed modulation, is used inthe optical interconnection system of the fifty-fifth embodiment, andbecause of this, high speed and large capacity optical transmission andoptical telecommunication become possible. More specifically, it shouldbe noted that the 1.3 μm band coincides to the zero dispersion band ofquartz in the case of the optical interconnection system constructed bythe surface-emission laser module in which a 1.3 μm bandsurface-emission laser diode that uses the GaInNAs mixed crystal for theactive layer is combined with a quartz single mode fiber as in the caseof the example 11 to be described later. Because of this, such aconstruction is well suited to high speed modulation. By using thissurface-emission laser module, it is possible to conduct high speed andlarge capacity optical telecommunication and optical transmission.

[1328] Also, it is possible to achieve stable oscillation up to hightemperatures even in the case there is caused a change of environmenttemperature in which the device is operated, in the case of thesurface-emission laser diode that uses the GaInNAs mixed semiconductorcrystal for the active, and because of this, it is possible to obtainhighly reliable optical interconnection system. Thus, the fifty-fifthembodiment provides a highly reliable optical interconnection systemcapable of performing high speed and large capacity opticalcommunication.

[1329] [Fifty-Sixth Embodiment]

[1330] A fifty-sixth embodiment of the present invention is an opticalcommunication system formed of the surface-emission laser diode of anyof the forty-fifth through fifty-second embodiments or asurface-emission laser array of the fifty-third embodiment or thesurface-emission laser module of the fifty-fourteenth embodiment.

[1331] As the optical telecommunication system of the fifty-sixthembodiment is formed of the surface-emission laser diode of any of theforty-fifth through fifty-second embodiments or the surface-emissionlaser array of the fifty-third embodiment or the surface-emission lasermodule of the fifty-fourth embodiment, it becomes possible to conducthigh speed and large capacity optical telecommunication.

[1332] Thus, a surface-emission laser diode or a surface-emission laserarray or a surface-emission laser module in which the electrostaticcapacitance is reduced in the n-type semiconductor distributed Braggreflector and capable of performing higher speed modulation, is used inthe optical telecommunication system of the fifty sixth embodiment, andbecause of this, high speed and large capacity optical telecommunicationbecome possible. More specifically, it should be noted that the 1.3 μmband coincides to the zero dispersion band of quartz in the case of theoptical telecommunication system constructed by the surface-emissionlaser module in which a 1.3 μm band surface-emission laser diode thatuses the GaInNAs mixed crystal for the active layer is combined with aquartz single mode fiber as in the case of the example 11 to bedescribed later. Because of this, such a construction is well suited tohigh speed modulation. By using this surface-emission laser module, itis possible to conduct high speed and large capacity opticaltransmission. Also, it is possible to achieve stable oscillation up tohigh temperatures even in the case there is caused a change ofenvironment temperature in which the device is operated, in the case ofthe surface-emission laser diode that uses the GaInNAs mixedsemiconductor crystal for the active, and because of this, it ispossible to obtain highly reliable optical telecommunication system.Thus, the fifty-sixth embodiment provides a highly reliable opticaltelecommunication system capable of performing high speed and largecapacity optical transmission.

EXAMPLES

[1333] Next, examples of the present invention will be described.

Example 5

[1334]FIG. 154 is a diagram explaining an example of the n-typesemiconductor distributed Bragg reflector of the forty-first embodimentas Example 5. The n-type semiconductor distributed Bragg reflector ofFIG. 154 is a semiconductor distributed Bragg reflector having a designreflection wavelength of 0.98 μm and is constructed by periodicallystacking an n-AlAs layer and n-GaAs layer with an intervening n-AlGaAslinear compositional graded layer (intermediate layer) of 30 nmthickness between the n-AlAs layer and the n-GaAs layer, wherein the Alcontent is changed linearly in the linear graded layer from a value toanother value as shown in FIG. 155.

[1335] In the n-type semiconductor distributed Bragg reflector of FIG.154, the thickness of the n-AlAs layer is set to 51.6 nm and thethickness of the n-GaAs layer is set to 40.9 nm. It should be noted thatthe thickness that satisfies the phase condition of multiple reflectionin the semiconductor distributed Bragg reflector, in other words thethickness in which the phase change of the becomes π/2 in each of theforegoing semiconductor layers, is 82.9 nm and 69.5 nm respectively forthe case of the light having the wavelength of 0.98 ,,m. However, thethickness of the n-AlAs layer and the n-GaAs layer are determined asabove in consideration of the phase change taking place in the linearcompositional graded layer having the thickness 30 nm.

[1336] The n-type semiconductor distributed Bragg reflector of FIG. 154can be formed by a crystal growth process conducted by an MOCVD processwhile using trimethyl aluminum (TMA), trimethyl gallium (TMG) and arsine(AsH₃) gas for gas source material. Thereby, it is possible to usehydrogen selenide (H₂Se) for the n-type dopant. In the MOVCD process,the composition of AlGaAs can be controlled easily by changing thesupply rate of the source materials, and thus, formation of thecompositional graded layer can be achieved easily.

[1337] In the n-type semiconductor distributed Bragg reflector of FIG.154, not only the electrical resistance but also the electrostaticcapacitance at the heterointerface are reduced as compared with aconventional device, by providing the compositional graded layer(intermediate layer) to each heterointerface.

Example 6

[1338]FIG. 171 is a diagram for explaining an example of the n-typesemiconductor distributed Bragg reflector of the forty-third embodimentas Example 6. It should be noted that the n-type semiconductordistributed Bragg reflector of FIG. 171 is a semiconductor distributedBragg reflector having a design reflection wavelength of 1.3 μm andincludes periodical stacking of an n-Al_(0.9)Ga0.1As layer and an n-GaAslayer with an n-AlGaAs linear compositional graded layer (intermediatelayer) of 40 nm thickness interposed between the n-Al_(0.9)Ga_(0.1)Aslayer and the n-GaAs layer, wherein the Al content of the linearcompositional graded layer is changed linearly from one value to anothervalue as shown in FIG. 155.

[1339] Here, it should be noted that the thickness of each of the layersis determined by reducing the thickness corresponding to the phasechange of light in the compositional graded layer similarly to Example5, such that the phase condition of the multiple reflection of the Braggreflector is satisfied Here, thickness of 40 nm of the compositionalgraded layer is chosen to fall in the ranges of 20<t≦(50λ−15) [nm],which is determined with respect to the reflection wavelength λ (=1.3μm) of the distributed Bragg reflector. With this, the electrostaticcapacitance by the heterointerface was successfully reduced as comparedwith conventional device while maintaining high reflectivity. Thus, ann-type distributed Bragg reflector suitable for the cavity reflector ofa surface-emission laser element, and the like, was obtained.

Example 7

[1340]FIG. 156 is a diagram showing an example of the surface-emissionlaser diode of the forty-fifth and fifty-second embodiments as Example7. It should be noted that the surface-emission laser diode of FIG. 156is a surface-emission laser diode of the 1.3 μm band that uses GaInNAsfor the active layer and is formed by conducting the crystal growth byan MOCVD process while using trimethyl aluminum (TMA), trimethyl gallium(TMG), trimethyl indium (TMI) and arsine gas (AsH₃) for the sourcematerial. Thereby, it is possible to use dimethylhydrazine (DMHy) forthe nitrogen source material of the active layer. Further, CBr₄ may beused for the p-type dopant and H₂Se for the n-type dopant.

[1341] The surface-emission laser diode of FIG. 156 is produced in thefollowing manner. First, crystal growth of an n-GaAs buffer layer isconducted on the n-GaAs substrate, and thereafter, crystal growth of then-type semiconductor distributed Bragg reflector is conducted byrepeating the AlAs/GaAs fundamental structure for 36 times. Thereby, then-AlGaAs linear compositional graded layer having the thickness of 30 nmis provided to each heterointerface of the n-AlAs/GaAs semiconductordistributed Bragg reflector as the intermediate layer (semiconductorlayer) as in the case of Example 5, such that that the Al contentchanges linearly from the composition of one of the semiconductor layersto the composition of the other semiconductor layer.

[1342] After forming the n-AlAs/GaAs semiconductor distributed Braggreflector like this, a GaAs cavity spacer layer, a GaInNAs/GaAs multiplequantum well structure (active layer), a GaAs cavity spacer layer, and ap-type semiconductor distributed Bragg reflector are formedconsecutively, wherein the p-type semiconductor distributed Braggreflector includes 22 stacks of the Al_(0.8)Ga_(0.2)As/GaAs pair. Here,it should be noted that there is provided an AlAs selective oxidationlayer of 30 nm thickness at the interface of the first pair in thep-type semiconductor distributed Bragg reflector. Further, in order toreduce the device resistance, there is provided a linear shapecompositional graded layer (thickness of 30 nm) at each interface of thep-type semiconductor distributed Bragg reflector such that the Alcontent changes linearly in the linear compositional graded layer from afirst composition at a first side to another composition at the oppositeside. Further, the GaAs layer forming the uppermost surface layer of thep-type semiconductor distributed Bragg reflector is used also for thecontact layer by setting the doping density to 1×10¹⁹ cm⁻³.

[1343] Here, it should be noted that the thickness of each layerconstituting the p-type semiconductor distributed Bragg reflector andthe n-type semiconductor distributed Bragg reflector here is adjustedsimilarly to Example 5 such that the phase condition of multiplereflection of the distributed Bragg reflector is satisfied, includingthe compositional graded layer (intermediate layer). Thus, adjustment ismade also to the thickness of the A1 _(0.8)Ga_(0.2)As layer adjacent tothe AlAs selective oxidation layer. Further, it should be noted that thephase change of the oscillation light in the region including the activelayer and also the two cavity spacer layers of this surface-emissionlaser diode is set equal to 2π. Thereby, there is formed 1λ cavity.Thereby, it should be noted that the active layer is disposed at thecentral position of the 1λ cavity of central in correspondence to theanti-node of the standing wave of light formed in the optical cavity.

[1344] Next, the layers from the surface of the p-GaAs contact layer upto midway of the GaAs cavity spacer layer contacting to the nsemiconductor distributed Bragg reflector are removed while leaving theregion constituting the device part in the form of mesa by conductingknown photolithographic process and dry etching process. The mesa thusformed as the device part has a square form of 30 μm×30 μm.

[1345] Next, heating is conducted in the ambient of bubbling pure waterof 80° C. formed by using a nitrogen gas, and a current confinementstructure is formed by conducting selective oxidation to as to proceedlaterally toward the device center from the etched sidewall of the AlAsselective oxidation layer. With this, a region serving for the currentpath is formed with the size of 5 μm×5 μm.

[1346] Next, the mesa part is buried by an insulating resin such aspolyimide, and evaporation deposition and lift-off of an electrodematerial is conducted to form a p-side electrode so as to form anopening for optical exit at the device top surface. Further, by formingthe n-side electrode to the rear surface of the substrate, it ispossible to obtain the surface-emission laser diode of FIG. 156.

[1347] In the surface-emission laser diode of FIG. 156, there isprovided the compositional graded layer at the heterointerface of bothof the p-type semiconductor distributed Bragg reflector and the n-typesemiconductor distributed Bragg reflector. It should be noted that thecompositional graded layer provided in the p-type semiconductordistributed Bragg reflector for reducing the device resistance as in thecase of the known technology. On the other hand, by providing the n-typecompositional graded layer in the n-type semiconductor distributed Braggreflector, the electrostatic capacitance of the device is reducedsignificantly as compared with a conventional device. In other words,the potential distribution such as spike, notch, and the like, at theinterface of the AlAs layer and the GaAs layer constituting the n-typesemiconductor distributed Bragg reflector is smoothed in thesurface-emission laser diode of FIG. 156 by providing the n-typecompositional graded layer to the n-type semiconductor distributed Braggreflector. With this, the accumulation or depletion of the carriers issuppressed and the electrostatic capacitance is reduced significantly.Thereby, a structure suitable for high speed modulation is obtained. Infact, the surface-emission laser diode of FIG. 156 is capable ofperforming high speed modulation of 10 Gbps or more easily.

[1348] Also, it should be noted that the surface-emission laser diode ofFIG. 156 makes use of GaInNAs for the active layer. With this, it becamepossible to provide a surface-emission laser diode oscillating at the1.3 μm by using the excellent semiconductor distributed Bragg reflectorthe having by the Al(Ga)As/GaAs structure on the GaAs substrate. Itshould be noted that the GaInNAs mixed crystal provides a largeconduction band discontinuity with regard to the barrier layer or cavityspacer layer of the GaAs, and the confinement effect of electrons to theactive layer is improved. Because of this, a stable oscillation isachieved up to high temperatures. Further, it should be noted that the1.3 μm band coincides with the zero dispersion band of silica fiber, andit becomes possible to achieve high speed communication by using aquartz single mode fiber. As noted above, high speed modulation ispossible in the surface-emission laser diode of FIG. 156. By combiningwith a quartz single mode fiber it becomes possible to conduct highspeed communication easily.

Example 8

[1349]FIG. 157 a diagram showing an example of the surface-emissionlaser diode of the forty-sixth and forty-eighth embodiment as Example 8.The surface-emission laser diode of FIG. 157 is a surface-emission laserdiode of the 0.98 ,,m band having an active layer of GaInAs/GaAsmultiple quantum well and crystal growth is made on the n-GaAs substrateby an MOCVD process.

[1350] Thus, in the surface-emission laser diode of FIG. 157, the n-typesemiconductor distributed Bragg reflectors is formed by repeatedlyconducting a crystal growth process of the Al_(0.8)Ga_(0.2)As/GaAs for36 times, and after this, the Al_(0.15)Ga_(0.85)As cavity spacer layer,the GaInAs/GaAs multiple quantum well structure, theAl_(0.15)Ga_(0.85)As cavity spacer layer, and the p-type semiconductordistributed Bragg reflector are formed consecutively, wherein the p-typesemiconductor distributed Bragg reflector includes therein 22 stacks ofthe Al_(0.8)Ga_(0.2)As/GaAs structure.

[1351] In the surface-emission laser diode of FIG. 157, it should benoted that the he linear compositional graded layer of the 30 nmthickness for reducing the device resistance is provided only in thep-type semiconductor distributed Bragg reflector. Further, it should benoted that an AlAs selective oxidation layer is provided similarly toExample 7 in the p-type semiconductor distributed Bragg reflector.

[1352] Further, the thickness of the layer constituting thesemiconductor distributed Bragg reflector is set so as to satisfy thephase condition of multiple reflection of the distributed Braggreflector for the light of the oscillation wavelength of 0.98 μm inconsideration of the thickness of the compositional graded layer and theselective oxidation layer.

[1353] Meanwhile, it should be noted that the path of the electrons inthe n-type semiconductor distributed Bragg reflector is limited to themesa region in the surface-emission laser diode of Example 8 in view ofthe fact that the n-type semiconductor distributed Bragg reflector isetched to the substrate surface (in other words, the n-typesemiconductor distributed Bragg reflector is processed and to form themesa by way of conducting etching (dry etching).

[1354] In a surface-emission laser diode, it should be noted that thedry etching is primarily the process conducted for preparation of thelateral oxidation of the selective oxidation layer, and because of this,it has been practiced in a conventional surface-emission laser diodethat the etching removal process is conducted only up to midway of then-type semiconductor distributed Bragg reflectors or midway of thecavity spacer layer or only for a few pairs of the layers in the n-typesemiconductor distributed Bragg reflector. Thus, in such a conventionalsurface-emission laser diode, the n-type semiconductor distributed Braggreflector does not function as the confinement structure of electrons,and the electron current is injected into the active region from then-side electrode was provided to the entire rear surface of thesubstrate through the n-type semiconductor distributed Bragg reflectorhaving an area larger than the mesa area. Thereby, the electrostaticcapacitance for the part of the interface of the n-type semiconductordistributed Bragg reflector acting as the path of the electrons currentbecomes the capacitance of the surface-emission laser diode.

[1355] Thus, by processing the n-type semiconductor distributed Braggreflector by etching to have the area comparable to the area of the mesaregion as in the case of Example 8 (by processing in the form of mesa),the electrostatic capacitance can be successfully reduced. Thus, thesurface-emission laser diode of Example 8 has an electrostaticcapacitance smaller than a conventional device and is well suited tohigh speed modulation. In fact, the surface-emission laser diode ofExample 8 was capable of performing high speed modulation exceeding 10Gbps easily.

[1356] Also, it becomes possible to reduce the capacitance of thesurface-emission laser diode more, by using the n-type semiconductordistributed Bragg reflector of FIG. 158 in the n-type semiconductordistributed Bragg reflector of the surface-emission laser diode of FIG.157. Thus, it should be noted that the n-type semiconductor distributedBragg reflector of FIG. 158 is the one in which the n-AlGaAs linearcompositional graded layer (intermediate layer) changing the Al contentlinearly from one side to the other side is provided to theheterointerface between the n-Al_(0.8)Ga_(0.2)As layer and the n-GaAslayer in the n-type semiconductor distributed Bragg reflector of FIG.154. By using the n-type semiconductor distributed Bragg reflector ofFIG. 158 for the n-type semiconductor distributed Bragg reflector of thesurface-emission laser diode of FIG. 157, the electrostatic capacitanceat the heterointerface is significantly reduced and the area of theregion serving for the electron current path is confined, andcontribution of such a region to the electrostatic capacitance isreduced. In this way, it becomes possible to reduce the devicecapacitance furthermore. Thus, by using the n-type semiconductordistributed Bragg reflector of FIG. 158 to a n-type semiconductordistributed Bragg reflector in the surface-emission laser diode of FIG.157, device structure extremely suitable for high speed modulation isobtained. In such a surface-emission laser diode, it was possible toconduct high speed modulation exceeding 10 Gbps easily.

Example 9

[1357]FIG. 159 is a diagram showing an example of the surface-emissionlaser diode of the forty-seventh and forty-eighth embodiment as Example9. The surface-emission laser diode of FIG. 159 is a surface-emissionlaser diode of the 0.98 μm band having an active layer of GaInAs/GaAsmultiple quantum well and crystal growth is made on the n-GaAs substrateby an MOCVD process.

[1358] Thus, in the surface-emission laser diode of FIG. 159, the n-typesemiconductor distributed Bragg reflectors is formed by repeatedlyconducting a crystal growth process of the AlAs/GaAs for 36 times, andafter this, the Al_(0.15)Ga_(0.85)As cavity spacer layer, theGaInAs/GaAs multiple quantum well structure, the Al_(0.15)Ga_(0.85)Ascavity spacer layer, and the p-type semiconductor distributed Braggreflector are formed consecutively, wherein the p-type semiconductordistributed Bragg reflector includes therein 22 stacks of theAl_(0.8)Ga_(0.2)As/GaAs structure.

[1359] In the surface-emission laser diode of

[1360]FIG. 159, it should be noted that the linear compositional gradedlayer of the 30 nm thickness for reducing the device resistance isprovided only in the p-type semiconductor distributed Bragg reflector.Further, it should be noted that an AlAs selective oxidation layer isprovided similarly to Example 8 in the p-type semiconductor distributedBragg reflector.

[1361] Further, the thickness of the layer constituting thesemiconductor distributed Bragg reflector is set so as to satisfy thephase condition of multiple reflection of the distributed Braggreflector for the light of the oscillation wavelength of 0.98 μm inconsideration of the thickness of the compositional graded layer and theselective oxidation layer.

[1362] Meanwhile, the surface-emission laser diode of Example 9 isformed by processing (dry etching) the layers starting from the surfaceof the p-GaAs contact layer up to the midway of the GaAs cavity spacerlayer adjacent to the n-semiconductor distributed Bragg reflector toform a mesa structure similarly to Example 7, and by conducting hydrogenion implantation into the n-type semiconductor distributed Braggreflector while using the resist pattern used at the time of the dryetching process as a mask, such that entire region of the n-typesemiconductor distributed Bragg reflectors where the ion implantationhas been made has a high resistance. In other words, the p-typesemiconductor distributed Bragg reflector has a high resistance exceptfor the mesa region (cavity region). After such ion implantation, theoxidation confinement structure is formed by the selective oxidationprocess of the AlAs layer. Further, the insulation resin is provided andthe p-side electrode and the n-side electrode are formed similarly toExample 7.

[1363] As explained with Example 8, the n-type semiconductor distributedBragg reflector does not form a confinement structure for the electronsin the conventional surface-emission laser diode, and thus, the currentpath of the electrons spreads beyond the mesa diameter in the n-typesemiconductor distributed Bragg reflector. Thus, the electrostaticcapacitance of the heterointerface in this part of the n-typesemiconductor distributed Bragg reflector contributes to the capacitanceof the surface-emission laser diode.

[1364] In the surface-emission laser diode of Example 9, on the otherhand, the electron current path is confined by providing high resistanceregion in the n-type semiconductor distributed Bragg reflector byconducting hydrogen ion implantation process, and because of this, itbecomes possible to eliminate unnecessary electrostatic capacitance.Thus, the surface-emission laser diode of Example 9 has a structuresuitable for high speed modulation. Further, because of formation of theconfinement structure of the electron current in the n-typesemiconductor distributed Bragg reflector by increasing the resistanceby hydrogen ion implantation. Because of this, there is no need ofincreasing the mesa height excessively, and the physical strength of thedevice is improved. Further, filling of the insulation resin can beconducted easily. Because of the fact that the distance between theactive region and the etched surface are near in the surface-emissionlaser diode of Example 9, there is no need of degrading the efficiencyof heat dissipation to the substrate, and decrease of output power byheat is easily reduced. In fact, it was possible to decrease theelectrostatic capacitance of the n-type semiconductor distributed Braggreflector in the surface-emission laser diode of Example 9. Thus, it waspossible to realize high speed modulation exceeding 10 Gbps easily.

[1365] Further, it became possible to reduce the capacity of thesurface-emission laser diode further, by using the n-type semiconductordistributed Bragg reflector of FIG. 160 for the n-type semiconductordistributed Bragg reflector of the surface-emission laser diode of FIG.159. Thus, the n-type semiconductor distributed Bragg reflector of FIG.160 is the one in which the n-AlGaAs linear compositional graded layer(intermediate layer) changing the Al content linearly from one sidethereof to the other said thereof is provided to the heterointerfacebetween the n-AlAs layer and the n-GaAs layer, similarly to the n-typesemiconductor distributed Bragg reflector of FIG. 154. By using then-type semiconductor distributed Bragg reflector of FIG. 160 as then-type semiconductor distributed Bragg reflector of the surface-emissionlaser diode of FIG. 159, the electrostatic capacitance caused by theheterointerface is can be reduced significantly. With this, the currentpath of the electrons is confined and the area contributing toelectrostatic capacitance is reduced. In this way, it becomes possibleto reduce the device capacitance furthermore, and a device structurewell suited for high speed modulation is obtained, by using the n-typesemiconductor distributed Bragg reflector of FIG. 160 for the n-typesemiconductor distributed Bragg reflector of the surface-emission laserdiode of FIG. 159. With such a surface-emission laser diode, it waspossible to conduct high speed modulation exceeding 10 Gbps easily.

[1366] In the aforementioned examples (Example 5, Example 7, Example 8,Example 9), the surface-emission laser diode formed by conducting acrystal growth process on an n-type semiconductor substrate was shown.However, it is also possible to use a p-type semiconductor substrateinstead of the n-type semiconductor substrate in the surface-emissionlaser diode of the present invention.

[1367] For example FIG. 161 is a diagram showing an example of realizingthe surface-emission laser diode of Example 7 on a p-type semiconductorsubstrate. It should be noted that the surface-emission laser diode ofFIG. 161 is formed by conducting a crystal growth process on a p-GaAssubstrate by an MOCVD process. More specifically, the surface-emissionlaser diode of FIG. 161 is formed by conducting a crystal growth to forma p-GaAs buffer layer on the p-GaAs substrate, followed by repeatedlygrowing the Al_(0.8)Ga_(0.2)As/GaAs structure for 36 stacks to form thep-type semiconductor distributed Bragg reflector. Thereafter, the GaAscavity spacer layer, the GaInNAs/GaAs multiple quantum well activelayer, the GaAs spacer layer, and the n-type semiconductor are formedconsecutively, wherein the n-type semiconductor is formed by stackingthe Al_(0.8)Ga_(0.2)As/GaAs fundamental structure for 26 times.

[1368] Here, it should be noted that a linear compositional graded layer(30 nm thickness) is provided to each of the heterointerfaces of thep-type semiconductor distributed Bragg reflector for reducing theelectrical resistance in such a manner to satisfy the phase condition ofmultiple reflection of the semiconductor distributed Bragg reflector.Further, the AlAs selective oxidation layer is provided to theAl_(0.8)Ga_(0.2)As/GaAs heterointerface nearest to the active layer inconformity with the phase condition. Further, the n-type semiconductordistributed Bragg reflector of the present invention is used for then-type semiconductor distributed Bragg reflector. Thus, a linearcompositional graded layer (30 nm thickness) is provided to eachheterointerface of the n-type semiconductor distributed Bragg reflectoras the intermediate layer (semiconductor layer) for reducing theelectrostatic capacitance.

[1369] In the surface-emission laser diode of FIG. 161, a dry etchingprocess, selective oxidation process, filling with the insulating resin,formation of the electrode, and the like, are conducted after thecrystal growth process, similarly to Example 7. Thereby, it should benoted that, in order to enable oxidation of the AlAs selective oxidationlayer in the p-type semiconductor distributed Bragg reflector, a dryetching process is conducted to the midway of the p-type semiconductordistributed Bragg reflector.

[1370] In the surface-emission laser diode of FIG. 161, it should benoted that high speed modulation exceeding 10 Gbps was achieved easilyas a result of the decrease of the electrostatic capacitance of then-type semiconductor distributed Bragg reflector similarly to Example 7.

[1371] Further, FIG. 162 is a diagram showing another example of thesurface-emission laser diode. The surface-emission laser diode of FIG.162 is produced similarly to the surface-emission laser diode of FIG.161 and there is provided a compositional graded layer in the p-typesemiconductor distributed Bragg reflector for reducing the electricalresistance. Further, such a compositional graded layer (intermediatelayer) is provided also in the n-type semiconductor distributed Braggreflector for reducing the electrostatic capacitance. In thesurface-emission laser diode of FIG. 162, it should be noted thathydrogen ion implantation is carried out similarly to Example 9 afterthe dry etching process for forming the mesa region. With this, theresistance of the p-type semiconductor distributed Bragg reflector isincreased except for the mesa region (cavity region). Thus, current pathof the holes is reduced to be generally equal to the mesa diameter, andthe electrostatic capacitance of the p-type semiconductor distributedBragg reflector is reduced. Thereby, the structure of FIG. 162 is suitedfor further high speed modulation as compared with the device of FIG.161.

[1372] In the examples (Example 5, Example 7, Example 8, Example 9)mentioned above, the crystal growth for producing the surface-emissionlaser diode was conducted by an MOCVD process However, it is alsopossible to conduct the crystal growth by using a growth method otherthan the MOCVD process. Further, in the above examples, a linearcompositional graded layer was used as the intermediate layer(semiconductor layer having the refractive index (the bandgap)intermediate of the two kinds of the semiconductor layers) providedbetween two kinds of semiconductor layers having respective, mutuallydifferent refractive indices and constituting the semiconductordistributed Bragg reflector, while it is also possible to use a layer inwhich the composition is changing to non-linearly for the intermediatelayer. Also, it is possible to use a layer formed of single or plurallayers having different refractive indices.

Example 10

[1373]FIG. 163 is a diagram showing an example of the surface-emissionlaser array of the fifty-third embodiment as Example 10. Thesurface-emission laser array of FIG. 163 is a monolithic laser array inwhich 3×3 surface-emission laser diodes of the present inventionsmentioned above are integrated in the form of two-dimensional array. Inthe surface-emission laser array of FIG. 163, separate p-side electrodewirings are provided for driving the individual surface-emission laserdiode independently. The surface-emission laser array of FIG. 163 isproduced with the procedure and method similar to those of Example 5,Example 7 or Example 9.

[1374] In the surface-emission laser array of FIG. 163, theelectrostatic capacitance caused by the n-type semiconductor distributedBragg reflector is reduced in each of the surface-emission laser diodesconstituting the surface-emission laser array. Accordingly, thissurface-emission laser array has a structure well suited to high speedmodulation, and in fact, high speed modulation was possible.

Example 11

[1375]FIG. 164 is a diagram showing an example of the surface-emissionlaser module of the fifty-fourth embodiment as Example 11. It can beseen that the surface-emission laser module of FIG. 164 is formed of aone-dimensional monolithic surface-emission laser array, a micro lensarray and a fiber array (quartz single mode fiber) mounted on siliconsubstrate.

[1376] Here, it should be noted that the surface-emission laser array ofthe fifty-third embodiment is provided so as to oppose the fibers, andthe surface-emission laser array is coupled with the quartz single modefibers mounted in the V-grooves formed on the silicon substrate by wayof a micro lens array. The oscillation wavelength of thissurface-emission laser array is in the 1.3 μm band. Thus, by using thequartz single mode fiber, it is possible to conduct high speed paralleloptical transmission.

[1377] Further, by using the surface-emission laser array of thefifty-third embodiment as the optical source of the surface-emissionlaser module of FIG. 164, high speed modulation becomes possible, and asurface-emission laser module capable of performing high speedtransmission can be obtained.

Example 12

[1378]FIG. 165 is a diagram showing the parallel optical interconnectionsystem as an example of the optical interconnection system of thefifty-fourth embodiment. In the inter connection system of FIG. 165, adevice 1 and a device 2 are connected by using an optical fiber array(quartz single mode fiber array). Here, it should be noted that thedevice 1 at the transmission side includes the surface-emission lasermodule formed by the surface-emission laser array of the fifty-fourthembodiment and a driver circuit thereof. Further, the device 2 at thereception side includes a photodiode array module and a signal detectioncircuit.

[1379] In the optical interconnection system of FIG. 165, high speedparallel optical transmission becomes possible by using thesurface-emission laser module of the fifty-fifth embodiment. Further, asurface-emission laser array having the active layer of GaInNAs is usedfor the surface-emission laser module. With this, it is possible toobtain stable oscillation even if there is caused change ofenvironmental temperature. As a result, an inter connection system ofvery high reliability can be constructed.

[1380] In an abovementioned example, explanation was made fro theexample of parallel optical interconnection system. However, in additionto this, it is possible to construct a serial transmission system thatuses a simple device. Further, the present-invention is not limited tothe interconnection between devices but the present invention isapplicable also to the inter connection between boards, between chips,or inside a chip.

Example 13

[1381]FIG. 166 is a diagram showing an optical LAN system as an exampleof the optical telecommunication system of the fifty-sixth embodiment.It should be noted that the optical LAN system of FIG. 166 isconstructed by the surface-emission laser diode or the surface-emissionlaser array or the surface-emission laser module of the presentinvention mentioned above.

[1382] Thus, in the optical LAN system of FIG. 166, the surface-emissionlaser diode or the surface-emission laser array or the surface-emissionlaser module of the present invention mentioned above is used for theoptical source of optical transmission between the core switch and theserver, and/or for the optical source of optical transmission betweenthe core switch and each switch, and/or for the optical source ofoptical transmission between the switch and each terminal. Further, aquartz single mode fiber or multimode fiber is used for interconnectionbetween the devices. A Giga-bit Ethernet of 1000BASE-LX, and the like,can be an example of the physical layer of such an optical LAN.

[1383] In the optical LAN system of FIG. 167, high speed communicationbecame possible as a result of use of the surface-emission laser diodeor the surface-emission laser array or the surface-emission laser moduleof the present invention that mentioned above for the optical source ofoptical transmission. Further, in the surface-emission laser of thepresent invention that uses GaInNAs for the active layer, stabileoscillation was obtained even if there is caused change of theoperational environmental temperature, drive condition, and the like,and it becomes possible to construct a highly reliable opticaltelecommunication system.

[1384] The present invention is not limited to the embodiments describedheretofore, but various variations and modifications may be made withoutdeparting from the scope of the invention.

[1385] The present application is based on Japanese priorityapplications No.2001-050145 filed on Feb. 26, 2001, No.2001-050171 filedon Feb. 26, 2001, No.2001-050083 filed on Feb. 26, 2001, No.2001-051253filed on Feb. 26, 2001, No.2001-051256 filed on Feb. 26, 2001,No.2001-051266 filed on Feb. 26, 2001, No.2001-053213 filed on Feb. 27,2001, No.2001-053218 filed on Feb. 27, 2001, No.2001-053200 filed onFeb. 27, 2001, No.2001-053190 filed on Feb. 27, 2001, No.2001-053225filed on Feb. 27, 2001, No.2001-073767 filed on Mar. 15, 2001,No.2001-090711 filed on Mar. 27, 2001, No.2002-019748 filed on Jan. 29,2002, No.2002-033590 filed on Feb. 12, 2002, No.2002-50548 filed on Feb.26, 2002, No.2003-118115 filed on Apr. 23, 2003, the entire contents ofwhich are hereby incorporated by reference.

What is claimed is:
 1. A distributed Bragg reflector, comprising: afirst semiconductor layer having a first, larger refractive index; asecond semiconductor layer having a second refractive index, said firstrefractive index larger than said second refractive index, said firstand second semiconductor layers being stacked alternately, and amaterial layer having a refractive index intermediate between said firstand second refractive indices, having a thickness equal to or largerthan 5 nm but equal to or smaller than 50 nm, and having a thirdrefractive index intermediate between said first and second refractiveindices, wherein said distributed Bragg reflector is tuned to awavelength of 1.1 μm or longer.
 2. A distributed Bragg reflector asclaimed in claim 1, wherein said material layer has a thickness equal toor larger than 20 nm.
 3. A distributed Bragg reflector as claimed inclaim 1, wherein said material layer has a thickness equal to or largerthan 30 nm.
 4. A distributed Bragg reflector as claimed in claim 2,wherein said first and second semiconductor layers are formed of any ofAlAs, GaAs and AlGaAs, and wherein there is a difference of Al contentof less than 80% between said first semiconductor layer and said secondsemiconductor layer.
 5. A distributed Bragg reflector as claimed inclaim 3, wherein said first semiconductor layer and said secondsemiconductor layer are formed of any of AlAs, GaAs and AlGaAs, andwherein there is a difference of Al content of 80% or more between saidfirst semiconductor layer and said second semiconductor layer.
 6. Adistributed Bragg reflector, comprising: a first semiconductor layerhaving a first refractive index; a second semiconductor layer having asecond refractive index, said first refractive index larger than saidsecond refractive index, said first and second semiconductor layersbeing stacked alternately; and a material layer having a thirdrefractive index intermediate between said first and second refractiveindices, said distributed Bragg reflector being tuned to a wavelength of1.1 μm or longer, said material layer having a thickness smaller than(50λ−15) (nm) where λ is a tuned wavelength of the distributed Braggreflector.
 7. A distributed Bragg reflector as claimed in claim 6,wherein said material layer has a thickness of 20 nm or more.
 8. Adistributed Bragg reflector as claimed in claim 6, wherein said materiallayer has a thickness of 30 nm or more.
 9. A distributed Braggreflector, comprising: a first semiconductor layer having a firstbandgap; a second semiconductor layer having a second bandgap, saidfirst bandgap smaller than said second bandgap, said first and secondsemiconductor layers being stacked alternately; and a material layerhaving a third bandgap intermediate between said first and secondbandgaps, provided between said first and second semiconductor layer,said material layer changing a valence band energy thereof in athickness direction from said first semiconductor layer to said secondsemiconductor layer, said material layer comprising a first layeradjacent to said first semiconductor layer and a second layer adjacentto said second semiconductor layer, and said first layer and secondlayer having first and second rates of compositional change such thatsaid first rate being larger than said second rate.
 10. A distributedBragg reflector as claimed in claim 9, wherein said intermediate layerchanges said valence band energy continuously and gradually from saidfirst semiconductor layer to said second semiconductor layer.
 11. Adistributed Bragg reflector as claimed in claim 9, wherein saidintermediate layer changes said valence band energy stepwise from saidfirst semiconductor layer to said second semiconductor layer.
 12. Adistributed Bragg reflector as claimed in claim 9, wherein saidintermediate layer comprises a layer in which said valence band energychanges continuously and a layer in which said valence band energychanges stepwise.
 13. A distributed Bragg reflector as claimed in claim9, wherein said first and second layers have respective first and secondthicknesses, such that said first thickness is smaller than said secondthickness.
 14. A distributed Bragg reflector as claimed in claim 9,wherein there is a stepped change of valence band energy at an interfacebetween said first semiconductor layer and said material layer.
 15. Adistributed Bragg reflector as claimed in claim 9, wherein said firstand second semiconductor layers comprise a material of AlGaAs system.16. A distributed Bragg reflector as claimed in claim 9, wherein saidfirst and second semiconductor layers comprise a material of AlGaInPsystem.
 17. A distributed Bragg reflector as claimed in claim 9, whereinsaid first and second semiconductor layers and said intermediate layerhave a carrier density of 5×10¹⁷ cm⁻³-2×10¹⁸ cm⁻³, said intermediatelayer has a thickness in the rage of 5-40 nm, and said intermediatelayer is characterized by an average change rate of Al content in therange of 0.02-0.15 nm⁻¹.
 18. A surface-emission laser diode, comprising:an active layer; and a resonator cooperating with said active layer,said active layer comprising upper and lower reflectors disposed aboveand below said active layer, at least one of said upper and lowerreflectors comprising a distributed Bragg reflector, comprising: a firstsemiconductor layer having a first refractive index; a secondsemiconductor layer having a second refractive index, said firstrefractive index larger than said second refractive index, said firstand second semiconductor layers being stacked alternately; and amaterial layer having a third refractive index intermediate between saidfirst and second refractive indices, said distributed Bragg reflectorbeing tuned to a wavelength of 1.1 μm or longer, said material layerhaving a thickness equal to or larger than 5 nm but equal to or smallerthan 50 nm.
 19. A surface-emission laser diode as claimed in claim 18,wherein said material layer has a thickness equal to or larger than 20nm.
 20. A surface-emission laser diode as claimed in claim 18, whereinsaid material layer has a thickness equal to or larger than 30 nm.
 21. Asurface-emission laser diode as claimed in claim 19, wherein said firstand second semiconductor layers are formed of any of AlAs, GaAs andAlGaAs, and wherein there is a difference of Al content of less than 80%between said first semiconductor layer and said second semiconductorlayer.
 22. A surface-emission laser diode as claimed in claim 20,wherein said first semiconductor layer and said second semiconductorlayer are formed of any of AlAs, GaAs and AlGaAs, and wherein there is adifference of Al content of 80% or more between said first semiconductorlayer and said second semiconductor layer.
 23. A surface-emission laserdiode as claimed in claim 18, wherein said active layer is formed of anyof a GaNAs layer, a GaInAs layer, a GaInNAs layer, a GaAsSb layer, aGaInAsSb layer, and a GaInNAsSb layer.
 24. A surface-emission laserdiode, comprising: an active layer; and a resonator cooperating withsaid active layer, said active layer comprising upper and lowerreflectors disposed above and below said active layer, at least one ofsaid upper and lower reflectors comprising a distributed Braggreflector, comprising: a first semiconductor layer having a firstrefractive index; a second semiconductor layer having a secondrefractive index, said first refractive index larger than said secondrefractive index, said first and second semiconductor layers beingstacked alternately; a material layer having a third refractive indexintermediate between said first and second refractive indices, saiddistributed Bragg reflector being tuned to a wavelength of 1.1 μm orlonger, said material layer having a thickness smaller than (50λ−15)(nm) where λ is a tuned wavelength of the distributed Bragg reflector.25. A surface-emission laser diode as claimed in claim 24, wherein saidmaterial layer has a thickness of 20 nm or more.
 26. A surface-emissionlaser diode as claimed in claim 24, wherein said material layer has athickness of 30 nm or more.
 27. A surface-emission laser diode asclaimed in claim 24, wherein said active layer is formed of any of aGaNAs layer, a GaInAs layer, a GaInNAs layer, a GaAsSb layer, a GaInAsSblayer, and a GaInNAsSb layer.
 28. A surface-emission laser diode,comprising: an active layer; and a resonator cooperating with saidactive layer, said active layer comprising upper and lower reflectorsdisposed above and below said active layer, at least one of said upperand lower reflectors comprising a distributed Bragg reflector,comprising: a first semiconductor layer having a first bandgap; a secondsemiconductor layer having a second bandgap, said first bandgap smallerthan said second bandgap, said first and second semiconductor layersbeing stacked alternately; and a material layer having a third bandgapintermediate between said first and second bandgaps, provided betweensaid first and second semiconductor layer, said material layer changinga valence band energy thereof in a thickness direction from said firstsemiconductor layer to said second semiconductor layer, said materiallayer comprising a first layer adjacent to said first semiconductorlayer and a second layer adjacent to said second semiconductor layer,and said first layer and second layer having first and second rates ofcompositional change such that said first rate being larger than saidsecond rate.
 29. A surface-emission laser diode as claimed in claim 28,wherein said intermediate layer changes said valence band energycontinuously and gradually from said first semiconductor layer to saidsecond semiconductor layer.
 30. A surface-emission laser diode asclaimed in claim 28, wherein said intermediate layer changes saidvalence band energy stepwise from said first semiconductor layer to saidsecond semiconductor layer.
 31. A surface-emission laser diode asclaimed in claim 28, wherein said intermediate layer comprises a layerin which said valence band energy changes continuously and a layer inwhich said valence band energy changes stepwise.
 32. A surface-emissionlaser diode as claimed in claim 28, wherein said first and second layershave respective first and second thicknesses, such that said firstthickness is smaller than said second thickness.
 33. A surface-emissionlaser diode as claimed in claim 28, wherein there is a stepped change ofvalence band energy at an interface between said first semiconductorlayer and said material layer.
 34. A surface-emission laser diode asclaimed in claim 28, wherein said first and second semiconductor layerscomprise a material of AlGaAs system.
 35. A surface-emission laser diodeas claimed in claim 28, wherein said first and second semiconductorlayers comprise a material of AlGaInP system.
 36. A surface-emissionlaser diode As claimed in claim 28, wherein said first and secondsemiconductor layers and said intermediate layer have a carrier densityof 5×10¹⁷ cm⁻³-2×10¹⁸ cm⁻³, and wherein said intermediate layer has athickness in the rage of 5-40 nm, and wherein said intermediate layer ischaracterized by an average change rate of Al content in the range of0.02-0.15 nm⁻¹.
 37. A laser diode array, comprising: a substrate; and aplurality of surface-emission laser diodes formed commonly on saidsubstrate, each of said plurality of surface-emission laser diodescomprising: an active layer; and a resonator cooperating with saidactive layer, said active layer comprising upper and lower reflectorsdisposed above and below said active layer, at least one of said upperand lower reflectors comprising a distributed Bragg reflector,comprising: a first semiconductor layer having a first refractive index;a second semiconductor layer having a second refractive index, saidfirst refractive index larger than said second refractive index, saidfirst and second semiconductor layers being stacked alternately; and amaterial layer having a third refractive index intermediate between saidfirst and second refractive indices, said distributed Bragg reflectorbeing tuned to a wavelength of 1.1 μm or longer, said material layerhaving a thickness equal to or larger than 5 nm but equal to or smallerthan 50 nm.
 38. A laser diode array, comprising: a substrate; and aplurality of surface-emission laser diodes formed commonly on saidsubstrate, each of said surface emission laser diodes comprising: anactive layer; and a resonator cooperating with said active layer, saidactive layer comprising upper and lower reflectors disposed above andbelow said active layer, at least one of said upper and lower reflectorscomprising a distributed Bragg reflector, comprising: a firstsemiconductor layer having a first refractive index; a secondsemiconductor layer having a second refractive index, said firstrefractive index larger than said second refractive index, said firstand second semiconductor layers being stacked alternately; and amaterial layer having a third refractive index intermediate between saidfirst and second refractive indices, said distributed Bragg reflectorbeing tuned to a wavelength of 1.1 μm or longer, said material layerhaving a thickness smaller than (50λ−15) (nm) where λ is a tunedwavelength of the distributed Bragg reflector.
 39. A surface-emissionlaser diode array, comprising: a substrate; and a plurality of laserdiodes, each of said surface-emission laser diodes, comprising: anactive layer; and a resonator cooperating with said active layer, saidactive layer comprising upper and lower reflectors disposed above andbelow said active layer, at least one of said upper and lower reflectorscomprising a distributed Bragg reflector, comprising: a firstsemiconductor layer having a first bandgap; a second semiconductor layerhaving a second bandgap, said first bandgap smaller than said secondbandgap, said first and second semiconductor layers being stackedalternately; and a material layer having a third bandgap intermediatebetween said first and second bandgaps, provided between said first andsecond semiconductor layer, said material layer changing a valence bandenergy thereof in a thickness direction from said first semiconductorlayer to said second semiconductor layer, said material layer comprisinga first layer adjacent to said first semiconductor layer and a secondlayer adjacent to said second semiconductor layer, and said first layerand second layer having first and second rates of compositional changesuch that said first rate being larger than said second rate.
 40. Anoptical interconnection system, comprising: a surface-emission laserdiode; and an optical transmission path coupled optically to saidsurface-emission laser diode, said surface-emission laser diodecomprising: an active layer; and a resonator cooperating with saidactive layer, said active layer comprising upper and lower reflectorsdisposed above and below said active layer, at least one of said upperand lower reflectors comprising a distributed Bragg reflector,comprising: a first semiconductor layer having a first refractive index;a second semiconductor layer having a second refractive index, saidfirst refractive index larger than said second refractive index, saidfirst and second semiconductor layers being stacked alternately; and amaterial layer having a third refractive index intermediate between saidfirst and second refractive indices, said distributed Bragg reflectorbeing tuned to a wavelength of 1.1 μm or longer, and said material layerhaving a thickness equal to or larger than 5 nm but equal to or smallerthan 50 nm.
 41. An optical interconnection system, comprising asurface-emission laser diode; and an optical transmission path coupledoptically to said surface-emission laser diode, said surface-emissionlaser diode comprising: an active layer; and a resonator cooperatingwith said active layer, said active layer comprising upper and lowerreflectors disposed above and below said active layer, at least one ofsaid upper and lower reflectors comprising a distributed Braggreflector, comprising: a first semiconductor layer having a firstrefractive index; a second semiconductor layer having a secondrefractive index, said first refractive index larger than said secondrefractive index, said first and second semiconductor layers beingstacked alternately; and a material layer having a third refractiveindex intermediate between said first and second refractive indices, andwherein there is provided a material layer having a refractive indexintermediate between said first refractive index and said secondrefractive index, said material layer having a thickness smaller than(50λ−15) (nm) where λ is a tuned wavelength of the distributed Braggreflector.
 42. An optical interconnection system, comprising: asurface-emission laser diode; and an optical transmission path coupledoptically to said surface-emission laser diode, said surface-emissionlaser diode comprising: an active layer; and a resonator cooperatingwith said active layer, said active layer comprising upper and lowerreflectors disposed above and below said active layer, at least one ofsaid upper and lower reflectors comprising a distributed Braggreflector, comprising: a first semiconductor layer having a firstbandgap; a second semiconductor layer having a second bandgap, saidfirst bandgap smaller than said second bandgap, said first and secondsemiconductor layers being stacked alternately; and a material layerhaving a third bandgap intermediate between said first and secondbandgaps, provided between said first and second semiconductor layer,said material layer changing a valence band energy thereof in athickness direction from said first semiconductor layer to said secondsemiconductor layer, and said material layer comprising a first layeradjacent to said first semiconductor layer and a second layer adjacentto said second semiconductor layer, said first layer and second layerhaving first and second rates of compositional change such that saidfirst rate being larger than said second rate.
 43. An opticalinterconnection system, comprising: a surface-emission laser diode arraycomprising a substrate and a plurality of surface-emission laser diodesprovided commonly on said substrate; and an optical transmission pathcoupled optically to each of said plurality of surface-emission laserdiodes, each of said plurality of surface-emission laser diodescomprising: an active layer; and a resonator cooperating with saidactive layer, said active layer comprising upper and lower reflectorsdisposed above and below said active layer, at least one of said upperand lower reflectors comprising a distributed Bragg reflector,comprising: a first semiconductor layer having a first refractive index;a second semiconductor layer having a second refractive index, saidfirst refractive index larger than said second refractive index, saidfirst and second semiconductor layers being stacked alternately; and amaterial layer having a third refractive index intermediate between saidfirst and second refractive indices, said distributed Bragg reflectorbeing tuned to a wavelength of 1.1 μm or longer, and said material layerhaving a thickness equal to or larger than 5 nm but equal to or smallerthan 50 nm.
 44. An optical interconnection system, comprising: asurface-emission laser diode array comprising a substrate and aplurality of surface-emission laser diodes formed commonly on saidsubstrate, and an optical transmission path coupled optically to each ofsaid plurality of surface-emission laser diodes, each of saidsurface-emission laser diodes comprising: an active layer; and aresonator cooperating with said active layer, said active layercomprising upper and lower reflectors disposed above and below saidactive layer, at least one of said upper and lower reflectors comprisinga distributed Bragg reflector, comprising: a first semiconductor layerhaving a first refractive index; a second semiconductor layer having asecond refractive index, said first refractive index larger than saidsecond refractive index, said first and second semiconductor layersbeing stacked alternately; and a material layer having a thirdrefractive index intermediate between said first and second refractiveindices, said distributed Bragg reflector being tuned to a wavelength of1.1 μm or longer, said material layer having a thickness smaller than(50λ−15) [nm] where λ is a tuned wavelength of the distributed Braggreflector.
 45. An optical interconnection system, comprising: asurface-emission laser diode array comprising a plurality ofsurface-emission laser diodes; and an optical transmission path coupledoptically to each of said plurality of surface-emission laser diodes,each of said surface-emission laser diodes comprising: an active layer;and a resonator cooperating with said active layer, said active layercomprising upper and lower reflectors disposed above and below saidactive layer, at least one of said upper and lower reflectors comprisinga distributed Bragg reflector, comprising: a first semiconductor layerhaving a first bandgap; a second semiconductor layer having a secondbandgap, said first bandgap smaller than said second bandgap, said firstand second semiconductor layers being stacked alternately; and amaterial layer having a third bandgap intermediate between said firstand second bandgaps, provided between said first and secondsemiconductor layer, said material layer changing a valence band energythereof in a thickness direction from said first semiconductor layer tosaid second semiconductor layer, said material layer comprising a firstlayer adjacent to said first semiconductor layer and a second layeradjacent to said second semiconductor layer, and said first layer andsecond layer having first and second rates of compositional change suchthat said first rate being larger than said second rate.
 46. An opticaltelecommunication system, comprising: a surface-emission laser diode;and an optical transmission path coupled optically to saidsurface-emission laser diode, said surface-emission laser diodecomprising: an active layer; and a resonator cooperating with saidactive layer, said active layer comprising upper and lower reflectorsdisposed above and below said active layer, at least one of said upperand lower reflectors comprising a distributed Bragg reflector,comprising; a first semiconductor layer having a first refractive index;a second semiconductor layer having a second refractive index, saidfirst refractive index larger than said second refractive index, saidfirst and second semiconductor layers being stacked alternately; and amaterial layer having a third refractive index intermediate between saidfirst and second refractive indices, said distributed Bragg reflectorbeing tuned to a wavelength of 1.1 μm or longer, and said material layerhaving a thickness equal to or larger than 5 nm but equal to or smallerthan 50 nm.
 47. An optical telecommunication system, comprising: asurface-emission laser diode; and an optical transmission path coupledoptically to said surface-emission laser diode, said surface-emissionlaser diode comprising; an active layer; and a resonator cooperatingwith said active layer, said active layer comprising upper and lowerreflectors disposed above and below said active layer, at least one ofsaid upper and lower reflectors comprising a distributed Braggreflector, comprising: a first semiconductor layer having a firstrefractive index; a second semiconductor layer having a secondrefractive index, said first refractive index larger than said secondrefractive index, said first and second semiconductor layers beingstacked alternately; and a material layer having a third refractiveindex intermediate between said first and second refractive indices,said distributed Bragg reflector being tuned to a wavelength of 1.1 μmor longer, and said material layer having a thickness smaller than(50λ−15) [nm] where λ is a tuned wavelength of the distributed Braggreflector.
 48. An optical telecommunication system, comprising: asurface-emission laser diode; and an optical transmission path coupledoptically to said surface-emission laser diode, said surface-emissionlaser diode comprising: an active layer; and a resonator cooperatingwith said active layer, said active layer comprising upper and lowerreflectors disposed above and below said active layer, at least one ofsaid upper and lower reflectors comprising a distributed Braggreflector, comprising: a first semiconductor layer having a firstbandgap; a second semiconductor layer having a second bandgap, saidfirst bandgap smaller than said second bandgap, said first and secondsemiconductor layers being stacked alternately; and a material layerhaving a third bandgap intermediate between said first and secondbandgaps, provided between said first and second semiconductor layer,said material layer changing a valence band energy thereof in athickness direction from said first semiconductor layer to said secondsemiconductor layer, said material layer comprising a first layeradjacent to said first semiconductor layer and a second layer adjacentto said second semiconductor layer, and said first layer and secondlayer having first and second rates of compositional change such thatsaid first rate being larger than said second rate.
 49. An opticaltelecommunication system, comprising: a surface-emission laser diodearray comprising a substrate and a plurality of surface-emission laserdiodes provided commonly on said substrate; and an optical transmissionpath coupled optically to each of said plurality of surface-emissionlaser diodes, each of said plurality of surface-emission laser diodescomprising: an active layer; and a resonator cooperating with saidactive layer, said active layer comprising upper and lower reflectorsdisposed above and below said active layer, at least one of said upperand lower reflectors comprising a distributed Bragg reflector,comprising: a first semiconductor layer having a first refractive index;a second semiconductor layer having a second refractive index, saidfirst refractive index larger than said second refractive index, saidfirst and second semiconductor layers being stacked alternately; and amaterial layer having a third refractive index intermediate between saidfirst and second refractive indices, said distributed Bragg reflectorbeing tuned to a wavelength of 1.1 μm or longer, and said material layerhaving a thickness equal to or larger than 5 nm but equal to or smallerthan 50 nm.
 50. An optical telecommunication system, comprising: asurface-emission laser diode array comprising a substrate and aplurality of surface-emission laser diodes formed commonly on saidsubstrate; and an optical transmission path coupled optically to each ofsaid plurality of surface-emission laser diodes, each of saidsurface-emission laser diodes comprising: an active layer; and aresonator cooperating with said active layer, said active layercomprising upper and lower reflectors disposed above and below saidactive layer, at least one of said upper and lower reflectors comprisinga distributed Bragg reflector, comprising: a first semiconductor layerhaving a first refractive index; a second semiconductor layer having asecond refractive index, said first refractive index larger than saidsecond refractive index, said first and second semiconductor layersbeing stacked alternately; and a material layer having a thirdrefractive index intermediate between said first and second refractiveindices, said distributed Bragg reflector being tuned to a wavelength of1.1 μm or longer, and said material layer having a thickness smallerthan (50λ−15) (nm) where λ is a tuned wavelength of the distributedBragg reflector.
 51. An optical telecommunication system, comprising: asurface-emission laser diode array comprising a plurality ofsurface-emission laser diodes; and an optical transmission path coupledoptically to each of said plurality of surface-emission laser diodes,each of said surface-emission laser diodes comprising: an active layer;and a resonator cooperating with said active layer, said active layercomprising upper and lower reflectors disposed above and below saidactive layer, at least one of said upper and lower reflectors comprisinga distributed Bragg reflector, comprising: a first semiconductor layerhaving a first bandgap; a second semiconductor layer having a secondbandgap, said first bandgap smaller than said second bandgap, said firstand second semiconductor layers being stacked alternately; and amaterial layer having a third bandgap intermediate between said firstand second band gaps, provided between said first and secondsemiconductor layer, said material layer changing a valence band energythereof in a thickness direction from said first semiconductor layer tosaid second semiconductor layer, said material layer comprising a firstlayer adjacent to said first semiconductor layer and a second layeradjacent to said second semiconductor layer, said first layer and secondlayer having first and second rates of compositional change such thatsaid first rate being larger than said second rate.
 52. An opticaltransmission/reception system, comprising: an optical source formed of asurface-emission laser diode device, said surface-emission laser diodecomprising: an active layer of any of a layer containing Ga, In, N andAs as major constituent elements thereof and a layer containing Ga, Inand As as major constituent elements thereof, said active layerproducing optical radiation with a laser oscillation wavelength of1.1-1.7 μm; and a cavity structure comprising a pair of reflectorsprovided above and below said active layer, each of said reflectorsforming a semiconductor distributed Bragg reflector reflecting opticalradiation having a wavelength of 1.1 μm or more and comprising analternate and repetitive stacking of a first material layer ofAl_(x)Ga_(1-x)As (0<x≦1) and a second material layer of Al_(y)Ga_(1-y)As(0≦y<x≦1), wherein there is provided a hetero spike buffer layer betweensaid first material layer and said second material layer, said heterospike buffer layer having a refractive index intermediate between arefractive index of said first material layer and a refractive index ofsaid second material layer, said hetero spike buffer layer having acomposition represented as AlzGa1-zAs (0≦y<z<x≦1) and a thickness of20-50 nm; an optical fiber transmission path having an end coupledoptically to said optical source; and a photodetection unit coupled tothe other end of said optical fiber transmission path, said opticalfiber transmission path being bent between a point A, in which saidoptical source is provided, and a point B, in which said photodetectionunit is provided, such that there is no localized angle formed in saidoptical fiber transmission path.
 53. An optical transmission/receptionsystem, comprising: an optical source formed of a surface-emission laserdiode device, said surface-emission laser diode comprising: an activelayer of any of a layer containing Ga, In, N and As as major constituentelements thereof and a layer containing Ga, In and As as majorconstituent elements thereof, said active layer producing opticalradiation with a laser oscillation wavelength of 1.1-1.7 μm; and acavity structure comprising a pair of reflectors provided above andbelow said active layer, each of said reflectors forming a semiconductordistributed Bragg reflector reflecting optical radiation having awavelength of 1.1 μm or more and comprising an alternate and repetitivestacking of a first material layer of Al_(x)Ga_(1-x)As (0<x≦1) and asecond material layer of Al_(y)Ga_(1-y)As (0≦y<x≦1), wherein there isprovided a hetero spike buffer layer between said first material layerand said second material layer, said hetero spike buffer layer having arefractive index intermediate between a refractive index of said firstmaterial layer and a refractive index of said second material layer,said hetero spike buffer layer having a composition represented asAlzGa1-zAs (0≦y<z<x≦1) and a thickness of 20-50 nm; an optical fibertransmission path having an end coupled to said optical source; aphotodetection unit coupled to another end of said optical fibertransmission path; and a mirror provided between a point A, in whichsaid optical source is provided, and a point B, in which saidphotodetection unit is provided, said mirror changing a direction ofpropagation of an optical signal transmitted through said optical fibertransmission path.
 54. An optical transmission/reception system for usein an apparatus, comprising: an apparatus body; a surface-emission laserdiode device provided in said apparatus body as a laser optical source,said laser optical source producing an optical signal; a photodetectionunit provided in said apparatus body, said photodetection unit receivingsaid optical signal; a cover member covering a light emitting part ofsaid laser optical source; and another cover member covering aphotodetection part of said photodetection unit, said surface-emissionlaser diode comprising: an active layer of any of a layer containing Ga,In, N and As as major constituent elements thereof and a layercontaining Ga, In and As as major constituent elements thereof, saidactive layer producing optical radiation with a laser oscillationwavelength of 1.1-1.7 μm; and a cavity structure comprising a pair ofreflectors provided above and below said active layer, each of saidreflectors forming a semiconductor distributed Bragg reflectorreflecting optical radiation having a wavelength of 1.1 μm or more andcomprising an alternate and repetitive stacking of a first materiallayer of Al_(x)Ga_(1-x)As (0<x≦1) and a second material layer ofAl_(y)Ga_(1-y)As (0≦y<x≦1), wherein there is provided a hetero spikebuffer layer between said first material layer and said second materiallayer, said hetero spike buffer layer having a refractive indexintermediate between a refractive index of said first material layer anda refractive index of said second material layer, said hetero spikebuffer layer having a composition represented as AlzGa1-zAs (0≦y<z<x≦1)and a thickness of 26-50 nm.
 55. An optical telecommunication system,comprising: a laser diode; a first optical fiber coupled optically tosaid laser diode, said first optical fiber being injected with a laserbeam produced by said laser diode; a second optical fiber coupledoptically to said first optical fiber, said second optical fiber beinginjected with an optical signal transmitted through said first opticalfiber; a third optical fiber coupled optically to said second opticalfiber, said third optical fiber being injected with an optical signaltransmitted through said second optical fiber; and a photodetectorcoupled optically to said third optical fiber, said photodetectordetecting an optical signal transmitted through said third opticalfiber, said laser diode comprising a surface-emission laser diode chipand comprising: an active layer of any of a layer containing Ga, In, Nand As as major constituent elements thereof and a layer containing Ga,In and As as major constituent elements thereof, said active layerproducing optical radiation with a laser oscillation wavelength of1.1-1.7 μm; and a cavity structure comprising a pair of reflectorsprovided above and below said active layer, each of said reflectorsforming a semiconductor distributed Bragg reflector reflecting opticalradiation having a wavelength of 1.1 μm or more and comprising analternate and repetitive stacking of a first material layer ofAl_(x)Ga_(1-x)As (0<x≦1) and a second material layer of Al_(y)Ga_(1-y)As(0≦y<x≦1), wherein there is provided a hetero spike buffer layer betweensaid first material layer and said second material layer, said heterospike buffer layer having a refractive index intermediate between arefractive index of said first material layer and a refractive index ofsaid second material layer, said hetero spike buffer layer having acomposition represented as AlzGa1-zAs (0≦y<z<x≦1) and a thickness of20-50 nm.
 56. An optical telecommunication system, comprising: a laserdiode; a first optical fiber coupled optically to said laser diode, saidfirst optical fiber being injected with a laser beam produced by saidlaser diode; a second optical fiber coupled optically to said firstoptical fiber, said second optical fiber being injected with an opticalsignal transmitted through said first optical fiber; a third opticalfiber coupled optically to said second optical fiber, said third opticalfiber being injected with an optical signal transmitted through saidsecond optical fiber, said laser diode comprising a surface-emissionlaser diode chip and comprising: an active layer of any of a layercontaining Ga, In, N and As as major constituent elements thereof and alayer containing Ga, In and As as major constituent elements thereof,said active layer producing optical radiation with a laser oscillationwavelength of 1.1-1.7 μm; and a cavity structure comprising a pair ofreflectors provided above and below said active layer, each of saidreflectors forming a semiconductor distributed Bragg reflectorreflecting optical radiation having a wavelength of 1.1 μm or more andcomprising an alternate and repetitive stacking of a first materiallayer of Al_(x)Ga_(1-x)As (0<x≦1) and a second material layer ofAl_(y)Ga_(1-y)As (0≦y<x≦1), wherein there is provided a hetero spikebuffer layer between said first material layer and said second materiallayer, said hetero spike buffer layer having a refractive indexintermediate between a refractive index of said first material layer anda refractive index of said second material layer, said hetero spikebuffer layer having a composition represented as AlzGa1-zAs (0≦y<z<x≦1)and a thickness of 20-50 nm, said first optical fiber having a length of1 mm or more.
 57. An optical telecommunication system comprising: alaser diode; and an optical transmission path coupled optically to saidlaser diode, said laser diode comprising a surface-emission laser diodechip and comprising: an active layer of any of a layer containing Ga,In, N and As as major constituent elements thereof and a layercontaining Ga, In and As as major constituent elements thereof, saidactive layer producing optical radiation with a laser oscillationwavelength of 1.1-1.7 μm; and a cavity structure comprising a pair ofreflectors provided above and below said active layer, each of saidreflectors forming a semiconductor distributed Bragg reflectorreflecting optical radiation having a wavelength of 1.1 μm or more andcomprising an alternate and repetitive stacking of a first materiallayer of Al_(x)Ga_(1-x)As (0<x≦1) and a second material layer ofAl_(y)Ga_(1-y)As (0≦y<x≦1), wherein there is provided a hetero spikebuffer layer between said first material layer and said second materiallayer, said hetero spike buffer layer having a refractive indexintermediate between a refractive index of said first material layer anda refractive index of said second material layer, said hetero spikebuffer layer having a composition represented as AlzGa1-zAs (0≦y<z<x≦1)and a thickness of 20-50 nm, said optical transmission path comprisingan optical fiber having a length L, said optical fiber including a corehaving a diameter D and a clad, wherein there holds a relationship 10³≦L/D≦10⁹.
 58. An optical telecommunication system, comprising: a laserdiode, a mount substrate on which said laser diode is mounted; saidlaser diode comprising a surface-emission laser diode chip andcomprising: an active layer of any of a layer containing Ga, In, N andAs as major constituent elements thereof and a layer containing Ga, Inand As as major constituent elements thereof, said active layerproducing optical radiation with a laser oscillation wavelength of1.1-1.7 μm; and a cavity structure comprising a pair of reflectorsprovided above and below said active layer, each of said reflectorsforming a semiconductor distributed Bragg reflector reflecting opticalradiation having a wavelength of 1.1 μm or more and comprising analternate and repetitive stacking of a first material layer ofAl_(x)Ga_(1-y)As (0<x≦1) and a second material layer of Al_(y)Ga_(1-y)As(0≦y<x≦1), wherein there is provided a hetero spike buffer layer betweensaid first material layer and said second material layer, said heterospike buffer layer having a refractive index intermediate between arefractive index of said first material layer and a refractive index ofsaid second material layer, said hetero spike buffer layer having acomposition represented as AlzGa1-zAs (0≦y<z<x≦1) and a thickness of20-50 nm, wherein a difference of linear thermal expansion coefficientbetween said laser diode and said substrate is within 2×10⁻⁶/K.
 59. Anoptical telecommunication system, comprising: a laser diode; and anoptical fiber coupled optically to said laser diode, said laser diodecomprising a surface-emission laser diode chip and comprising: an activelayer of any of a layer containing Ga, In, N and As as major constituentelements thereof and a layer containing Ga, In and As as majorconstituent elements thereof, said active layer producing opticalradiation with a laser oscillation wavelength of 1.1-1.7 μm; and acavity structure comprising a pair of reflectors provided above andbelow said active layer, each of said reflectors forming a semiconductordistributed Bragg reflector reflecting optical radiation having awavelength of 1.1 μm or more and comprising an alternate and repetitivestacking of a first material layer of Al_(x)Ga_(1-x)As (0<x≦1) and asecond material layer of Al_(y)Ga_(1-y)As (0≦y<x≦1), wherein there isprovided a hetero spike buffer layer between said first material layerand said second material layer, said hetero spike buffer layer having arefractive index intermediate between a refractive index of said firstmaterial layer and a refractive index of said second material layer,said hetero spike buffer layer having a composition represented asAlzGa1-zAs (0≦y<z<x≦1) and a thickness of 20-50 nm, wherein said opticalfiber is mechanically connected to said laser diode in the state thatsaid optical fiber is urged in an axial direction thereof toward a lightemitting part of said laser diode.
 60. An optical telecommunicationsystem, comprising: a laser diode; and one of an optical fiber and anoptical waveguide coupled optically to said laser diode, said laserdiode comprising a surface-emission laser diode chip and comprising: anactive layer of any of a layer containing Ga, In, N and As as majorconstituent elements thereof and a layer containing Ga, In and As asmajor constituent elements thereof, said active layer producing opticalradiation with a laser oscillation wavelength of 1.1-1.7 μm; and acavity structure comprising a pair of reflectors provided above andbelow said active layer, each of said reflectors forming a semiconductordistributed Bragg reflector reflecting optical radiation having awavelength of 1.1 μm or more and comprising an alternate and repetitivestacking of a first material layer of Al_(x)Ga_(1-x)As (0<x≦1) and asecond material layer of Al_(y)Ga_(1-y)As (0≦y<x≦1), wherein there isprovided a hetero spike buffer layer between said first material layerand said second material layer, said hetero spike buffer layer having arefractive index intermediate between a refractive index of said firstmaterial layer and a refractive index of said second material layer,said hetero spike buffer layer having a composition represented asAlzGa1-zAs (0≦y<z<x≦1) and a thickness of 20-50 nm, said optical fiberor said optical waveguide having a core with a diameter x, said laserdiode having an aperture d and an optical emission angle θ, whereinthere holds a relationship d+21 tan(θ/2)≦x, where 1 represents anoptical path length from said laser diode to an edge of said opticalfiber or optical waveguide.
 61. An optical telecommunication system,comprising: a laser diode; and an optical waveguide coupled optically tosaid laser diode, said laser diode comprising a surface-emission laserdiode chip and comprising: an active layer of any of a layer containingGa, In, N and As as major constituent elements thereof and a layercontaining Ga, In and As as major constituent elements thereof, saidactive layer producing optical radiation with a laser oscillationwavelength of 1.1-1.7 μm; and a cavity structure comprising a pair ofreflectors provided above and below said active layer, each of saidreflectors forming a semiconductor distributed Bragg reflectorreflecting optical radiation having a wavelength of 1.1 μm or more andcomprising an alternate and repetitive stacking of a first materiallayer of Al_(x)Ga_(1-x)As (0<x≦1) and a second material layer ofAl_(y)Ga_(1-y)As (0≦y<x≦1), wherein there is provided a hetero spikebuffer layer between said first material layer and said second materiallayer, said hetero spike buffer layer having a refractive indexintermediate between a is refractive index of said first material layerand a refractive index of said second material layer, said hetero spikebuffer layer having a composition represented as AlzGa1-zAs (0≦y<z<x≦1)and a thickness of 20-50 nm, wherein there holds a relationship0.5≦F/d≦2 where d represents a diameter of a circle touching internallyto an optical emission part of said laser diode and F represents a corediameter of said optical fiber.
 62. An optical telecommunication system,comprising: a laser diode; and an optical waveguide coupled optically toa laser chip, said laser diode comprising a surface-emission laser diodechip and comprising: an active layer of any of a layer containing Ga,In, N and As as major constituent elements thereof and a layercontaining Ga, In and As as major constituent elements thereof, saidactive layer producing optical radiation with a laser oscillationwavelength of 1.1-1.7 μm; and a cavity structure comprising a pair ofreflectors provided above and below said active layer, each of saidreflectors forming a semiconductor distributed Bragg reflectorreflecting optical radiation having a wavelength of 1.1 μm or more andcomprising an alternate and repetitive stacking of a first materiallayer of Al_(x)Ga1-As (0<x≦1) and a second material layer ofAl_(y)Ga_(1-y)As (0≦y<x≦1), wherein there is provided a hetero spikebuffer layer between said first material layer and said second materiallayer, said hetero spike buffer layer having a refractive indexintermediate between a refractive index of said first material layer anda refractive index of said second material layer, said hetero spikebuffer layer having a composition represented as AlzGa1-zAs (0≦y<z<x≦1)and a thickness of 20-50 nm, said laser diode including an opticalemission part having an area S [mm²], said laser diode being driven withan operational voltage V [volts], wherein a parameter V/S falls in arange from 15000 to
 30000. 63. A semiconductor distributed Braggreflector comprising: an alternate stacking of first and secondsemiconductor layers having respective, different refractive indices;and a plurality of intermediate layers each sandwiched between a firstsemiconductor layer and a second semiconductor layer, said intermediatelayer having a refractive index intermediate between said refractiveindices of said first and second semiconductor layers, an intermediatelayer provided in a region of said semiconductor distributed Braggreflector having a thickness different from an intermediate layerprovided in a different region of said semiconductor distributed Braggreflector.
 64. The semiconductor distributed Bragg reflector as claimedin claim 63, wherein a difference of bandgap between said first andsecond semiconductor layers is set smaller in a region of saidsemiconductor distributed Bragg reflector where said intermediate layerhas an increased thickness than in a region of said distributed Braggreflector where said intermediate layer has a reduced thickness.
 65. Thesemiconductor distributed Bragg reflector as claimed in claim 63,wherein said intermediate layers have different thickness and differentdoping concentrations within said semiconductor distributed Braggreflector, said thickness and doping concentration being changed incorrespondence to electric field strength of light within saidsemiconductor distributed Bragg reflector.
 66. The semiconductordistributed Bragg reflector as claimed in claim 65, wherein saidintermediate layer has an increased thickness and reduced impuritydoping concentration in a region of said semiconductor distributed Braggreflector where the electric field strength of light is large, andwherein said intermediate layer is formed to have a reduced thicknessand increased impurity doping concentration in a region of saidsemiconductor distributed Bragg reflector where the electric fieldstrength of light is small.
 67. The semiconductor distributed Braggreflector as claimed in claim 63, wherein said semiconductor distributedBragg reflector has a design reflection wavelength of 1.1 μm or longer.68. A surface-emission laser diode having a semiconductor distributedBragg reflector, said semiconductor distributed Bragg reflectorcomprising: an alternate stacking of first and second semiconductorlayers having respective, different refractive indices; and a pluralityof intermediate layers each sandwiched between a first semiconductorlayer and a second semiconductor layer, said intermediate layer having arefractive index intermediate between said refractive indices of saidfirst and second semiconductor layers, an intermediate layer provided ina region of said semiconductor distributed Bragg reflector having athickness different from an intermediate layer provided in a differentregion of said semiconductor distributed Bragg reflector.
 69. Thesurface-emission laser diode as claimed in claim 68, wherein adifference of bandgap between said first and second semiconductor layersis set smaller in a region of said semiconductor distributed Braggreflector where said intermediate layer has an increased thickness thanin a region of said distributed Bragg reflector where said intermediatelayer has a reduced thickness.
 70. The surface-emission laser diode asclaimed in claim 68, wherein said intermediate layers have differentthickness and different doping concentrations within said semiconductordistributed Bragg reflector, said thickness and doping concentrationbeing changed in correspondence to electric field strength of lightwithin said semiconductor distributed Bragg reflector.
 71. Thesurface-emission laser diode as claimed in claim 68, wherein saidintermediate layer has an increased thickness and reduced impuritydoping concentration in a region of said semiconductor distributed Braggreflector where the electric field strength of light is large, andwherein said intermediate layer is formed to have a reduced thicknessand increased impurity doping concentration in a region of saidsemiconductor distributed Bragg reflector where the electric fieldstrength of light is small.
 72. The surface-emission laser diode asclaimed in claim 68, wherein said semiconductor distributed Braggreflector has a design reflection wavelength of 1.1 μm or longer. 73.The surface-emission laser diode as claimed in claim 68, wherein saidactive layer contains a group III element of any or all of Ga and In anda group V element of any or all of As, N and Sb.
 74. A surface-emissionlaser array including a plurality of surface-emission laser diodes eachhaving a semiconductor distributed Bragg reflector, said semiconductordistributed Bragg reflector comprising; an alternate stacking of firstand second semiconductor layers having respective, different refractiveindices; and a plurality of intermediate layers each sandwiched betweena first semiconductor layer and a second semiconductor layer, saidintermediate layer having a refractive index intermediate between saidrefractive indices of said first and second semiconductor layers, anintermediate layer provided in a region of said semiconductordistributed Bragg reflector having a thickness different from anintermediate layer provided in a different region of said semiconductordistributed Bragg reflector.
 75. The surface-emission laser array asclaimed in claim 74, wherein a difference of bandgap between said firstand second semiconductor layers is set smaller in a region of saidsemiconductor distributed Bragg reflector where said intermediate layerhas an increased thickness than in a region of said distributed Braggreflector where said intermediate layer has a reduced thickness.
 76. Thesurface-emission laser array as claimed in claim 74, wherein saidintermediate layers have different thickness and different dopingconcentrations within said semiconductor distributed Bragg reflector,said thickness and doping concentration being changed in correspondenceto electric field strength of light within said semiconductordistributed Bragg reflector.
 77. The semiconductor distributed Braggreflector as claimed in claim 76, wherein said intermediate layer has anincreased thickness and reduced impurity doping concentration in aregion of said semiconductor distributed Bragg reflector where theelectric field strength of light is large, and wherein said intermediatelayer is formed to have a reduced thickness and increased impuritydoping concentration in a region of said semiconductor distributed Braggreflector where the electric field strength of light is small.
 78. Thesurface-emission laser array as claimed in claim 74, wherein saidsemiconductor distributed Bragg reflector has a design reflectionwavelength of 1.1 μm or longer.
 79. A surface-emission laser moduleincluding a surface-emission laser diode having a semiconductordistributed Bragg reflector, said semiconductor distributed Braggreflector comprising: an alternate stacking of first and secondsemiconductor layers having respective, different refractive indices;and a plurality of intermediate layers each sandwiched between a firstsemiconductor layer and a second semiconductor layer, said intermediatelayer having a refractive index intermediate between said refractiveindices of said first and second semiconductor layers, an intermediatelayer provided in a region of said semiconductor distributed Braggreflector having a thickness different from an intermediate layerprovided in a different region of said semiconductor distributed Braggreflector.
 80. A surface-emission laser module as claimed in claim 79,wherein said surface-emission laser diode is provided in plural numberin the form of an array.
 81. An optical interconnection system includinga surface-emission laser diode having a semiconductor distributed Braggreflector, said semiconductor distributed Bragg reflector comprising: analternate stacking of first and second semiconductor layers havingrespective, different refractive indices; and a plurality ofintermediate layers each sandwiched between a first semiconductor layerand a second semiconductor layer, said intermediate layer having arefractive index intermediate between said refractive indices of saidfirst and second semiconductor layers, an intermediate layer provided ina region of said semiconductor distributed Bragg reflector having athickness different from an intermediate layer provided in a differentregion of said semiconductor distributed Bragg reflector.
 82. An opticalinterconnection system as claimed in claim 77, wherein saidsurface-emission laser diode is provided in plural number in the form ofa surface-emission laser array.
 83. An optical interconnection system asclaimed in claim 81, wherein said surface-emission laser diode forms asurface-emission laser module.
 84. An optical telecommunication systemincluding a surface-emission laser diode having a semiconductordistributed Bragg reflector, said semiconductor distributed Braggreflector comprising: an alternate stacking of first and secondsemiconductor layers having respective, different refractive indices;and a plurality of intermediate layers each sandwiched between a firstsemiconductor layer and a second semiconductor layer, said intermediatelayer having a refractive index intermediate between said refractiveindices of said first and second semiconductor layers, an intermediatelayer provided in a region of said semiconductor distributed Braggreflector having a thickness different from an intermediate layerprovided in a different region of said semiconductor distributed Braggreflector.
 85. An optical interconnection system as claimed in claim 84,wherein said surface-emission laser diode is provided in plural numberin the form of a surface-emission laser array.
 86. An opticaltelecommunication system as claimed in claim 84, wherein saidsurface-emission laser diode forms a surface-emission laser module. 87.An n-type semiconductor distributed Bragg reflector comprising: firstand second semiconductor layers of n-type stacked with each other, saidfirst and second semiconductor layers having respective refractiveindices different from each other; and an intermediate layer providedbetween said first and second semiconductor layers, said intermediatelayer having a refractive index intermediate of said refractive indicesof said first and second semiconductor layers.
 88. An n-typesemiconductor distributed Bragg reflector as claimed in claim 87,wherein said intermediate layer has a thickness larger than 20 [nm] insaid n-type semiconductor distributed Bragg reflector.
 89. An n-typesemiconductor distributed Bragg reflector as claimed in claim 87,wherein said intermediate layer has a thickness equal to or larger than30 [nm] in said n-type semiconductor distributed Bragg reflector.
 90. Ann-type semiconductor distributed Bragg reflector as claimed in claim 87,wherein said intermediate layer has a thickness t [nm] determined withrespect to a reflection wavelength λ [um] of said distributed Braggreflector so as to fall in the ranges of 20<t≦(50λ−15) [nm].
 91. Asurface-emission laser diode having an n-type semiconductor distributedBragg reflector, said n-type semiconductor distributed Bragg reflectorcomprising: first and second semiconductor layers of n-type stacked witheach other, said first and second semiconductor layers having respectiverefractive indices different from each other; and an intermediate layerprovided between said first and second semiconductor layers, saidintermediate layer having a refractive index intermediate of saidrefractive indices of said first and second semiconductor layers.
 92. Asurface-emission laser diode as claimed in claim 91, wherein saidintermediate layer has a thickness larger than 20 [nm] in said n-typesemiconductor distributed Bragg reflector.
 93. A surface-emission laserdiode as claimed in claim 91 wherein said intermediate layer has athickness equal to or larger than 30 [nm] in said n-type semiconductordistributed Bragg reflector.
 94. A surface-emission laser diode asclaimed in claim 91, wherein said intermediate layer has a thickness t[nm] determined with respect to a reflection wavelength λ [um] of saiddistributed Bragg reflector so as to fall in the ranges of 20<t≦(50λ−15)[nm].
 95. A surface-emission laser diode as claimed in claim 91, whereinsaid active layer is formed of a group III element and a group Velement, said group III element of said active layer being any or all ofGa and In, said group V element of said active layer being any or all ofAs, N, Sb and P.
 96. A surface-emission laser diode, comprising: anactive layer; an n-type semiconductor distributed Bragg reflector; and ap-type semiconductor distributed Bragg reflector, said n-typesemiconductor distributed Bragg reflector and said p-type semiconductordistributed Bragg reflector being disposed at both sides of said activelayer, wherein said n-type semiconductor distributed Bragg reflector isprocessed to form a mesa.
 97. A surface-emission laser diode as claimedin claim 96, wherein said n-type semiconductor distributed Braggreflector comprises stacking of first and second semiconductor layershaving respective, mutually different refractive indices, said n-typesemiconductor distributed Bragg reflector further comprises anintermediate layer having a refractive index intermediate of said firstand second semiconductor layer, between said first and secondsemiconductor layers.
 98. A surface-emission laser diode as claimed inclaim 96, wherein said n-type semiconductor distributed Bragg reflectorcomprises stacking of first and second semiconductor layers havingrespective refractive indices different from each other, said n-typesemiconductor distributed Bragg reflector further including anintermediate layer having a refractive index intermediate of saidrefractive indices of said first and second semiconductor layers betweensaid first and second semiconductor layers with a thick larger than 20[nm].
 99. A surface-emission laser diode as claimed in claim 96, whereinsaid n-type semiconductor distributed Bragg reflector comprises stackingof a first and second semiconductor layers having respective refractiveindices different from each other, said n-type semiconductor distributedBragg reflector further including an intermediate layer having arefractive index intermediate of said first and second semiconductorlayers between said first and second semiconductor layers with athickness of 30 [nm] or more.
 100. A surface-emission laser diode asclaimed in claim 96, wherein said n-type semiconductor distributed Braggreflector comprises stacking of first and second semiconductor layershaving respective refractive indices different from each other, saidn-type semiconductor distributed Bragg reflector further including anintermediate layer having a refractive index intermediate of said firstand second semiconductor layers, between said first and secondsemiconductor layers with a thickness t [nm] determined with respect toa reflection wavelength λ [um] of said distributed Bragg reflector so asto fall in the ranges of 20<t≦(50λ−15) [nm].
 101. A surface-emissionlaser diode as claimed in claim 96, wherein said active layer is formedof a group III element and a group v element, said group III element ofsaid active layer being any or all of Ga and In, said group V element ofsaid active layer being any or all of As, N, Sb and P.
 102. Asurface-emission laser diode comprising: an active layer; an n-typesemiconductor distributed Bragg reflector; and a p-type semiconductordistributed Bragg reflector, said n-type semiconductor distributed Braggreflector and said p-type semiconductor distributed Bragg reflectorbeing disposed at both sides of said active layer, said n-typesemiconductor distributed Bragg reflector having an increased resistancewith respect to a region forming a cavity of said the surface-emissionlaser diode.
 103. A surface-emission laser diode as claimed in claim102, wherein said n-type semiconductor distributed Bragg reflectorcomprises stacking of first and second semiconductor layers havingrespective, mutually different refractive indices, said n-typesemiconductor distributed Bragg reflector further comprising anintermediate layer having a refractive index intermediate of said firstand second semiconductor layer, between said first and secondsemiconductor layers.
 104. A surface-emission laser diode as claimed inclaim 102, wherein said n-type semiconductor distributed Bragg reflectorcomprises stacking of first and second semiconductor layers havingrespective refractive indices different from each other, said n-typesemiconductor distributed Bragg reflector further including anintermediate layer having a refractive index intermediate of saidrefractive indices of said first and second semiconductor layers betweensaid first and second semiconductor layers with a thick larger than 20[nm].
 105. A surface-emission laser diode as claimed in claim 102,wherein said n-type semiconductor distributed Bragg reflector comprisesstacking of a first and second semiconductor layers having respectiverefractive indices different from each other, said n-type semiconductordistributed Bragg reflector further including an intermediate layerhaving a refractive index intermediate of said first and secondsemiconductor layers between said first and second semiconductor layerswith a thickness of 30 [nm] or more.
 106. A surface-emission laser diodeas claimed in claim 102, wherein said n-type semiconductor distributedBragg reflector comprises stacking of first and second semiconductorlayers having respective refractive indices different from each other,said n-type semiconductor distributed Bragg reflector further includingan intermediate layer having a refractive index intermediate of saidfirst and second semiconductor layers, between said first and secondsemiconductor layers with a thickness t [nm] determined with respect toa reflection wavelength λ [nm] of said distributed Bragg reflector so asto fall in the ranges of 20<t≦(50λ−15) [nm].
 107. A surface-emissionlaser diode as claimed in claim 102, wherein said active layer is formedof a group III element and a group V element, said group III element ofsaid active layer being any or all of Ga and In, said group V element ofsaid active layer being any or all of As, N, Sb and P.
 108. Asurface-emission laser array including a surface-emission laser diode,said surface-emission laser diode having an n-type semiconductordistributed Bragg reflector, said n-type semiconductor distributed Braggreflector comprising: first and second semiconductor layers of n-typestacked with each other, said first and second semiconductor layershaving respective refractive indices different from each other; and anintermediate layer provided between said first and second semiconductorlayers, said intermediate layer having a refractive index intermediateof said refractive indices of said first and second semiconductorlayers.
 109. A surface-emission laser array as claimed in claim 108,wherein said intermediate layer has a thickness larger than 20 [nm] insaid n-type semiconductor distributed Bragg reflector.
 110. Asurface-emission laser array as claimed in claim 108, wherein saidintermediate layer has a thickness equal to or larger than 30 [nm] insaid n-type semiconductor distributed Bragg reflector.
 111. Asurface-emission laser array as claimed in claim 108, wherein saidintermediate layer has a thickness t [nm] determined with respect to areflection wavelength λ [um] of said distributed Bragg reflector so asto fall in the ranges of 20<t≦(50λ−15) [nm].
 112. A surface-emissionlaser array including a surface-emission laser diode, saidsurface-emission laser diode, comprising: an active layer; an n-typesemiconductor distributed Bragg reflector; and a p-type semiconductordistributed Bragg reflector, said n-type semiconductor distributed Braggreflector and said p-type semiconductor distributed Bragg reflectorbeing disposed at both sides of said active layer, wherein said n-typesemiconductor distributed Bragg reflector is processed to form a mesa.113. A surface-emission laser array as claimed in claim 112, whereinsaid intermediate layer has a thickness larger than 20 [nm] in saidn-type semiconductor distributed Bragg reflector.
 114. Asurface-emission laser array as claimed in claim 112, wherein saidintermediate layer has a thickness equal to or larger than 30 [nm] insaid n-type semiconductor distributed Bragg reflector.
 115. Asurface-emission laser array as claimed in claim 112, wherein saidintermediate layer has a thickness t [nm] determined with respect to areflection wavelength λ [um] of said distributed Bragg reflector so asto fall in the ranges of 20<t≦(50λ−15) [nm].
 116. A surface-emissionlaser array including a surface-emission laser diode, saidsurface-emission laser diode comprising: an active layer; an n-typesemiconductor distributed Bragg reflector; and a p-type semiconductordistributed Bragg reflector, said n-type semiconductor distributed Braggreflector and said p-type semiconductor distributed Bragg reflectorbeing disposed at both sides of said active layer, said n-typesemiconductor distributed Bragg reflector having an increased resistancewith respect to a region forming a cavity of said the surface-emissionlaser diode.
 117. A surface-emission laser module including asurface-emission laser diode, said surface-emission laser diode havingan n-type semiconductor distributed Bragg reflector, said n-typesemiconductor distributed Bragg reflector comprising: first and secondsemiconductor layers of n-type stacked with each other, said first andsecond semiconductor layers having respective refractive indicesdifferent from each other; and an intermediate layer provided betweensaid first and second semiconductor layers, said intermediate layerhaving a refractive index intermediate of said refractive indices ofsaid first and second semiconductor layers.
 118. A surface-emissionlaser module including a surface-emission laser diode, saidsurface-emission laser diode, comprising: an active layer; an n-typesemiconductor distributed Bragg reflector; and a p-type semiconductordistributed Bragg reflector, said n-type semiconductor distributed Braggreflector and said p-type semiconductor distributed Bragg reflectorbeing disposed at both sides of said active layer, wherein said n-typesemiconductor distributed Bragg reflector is processed to form a mesa.119. A surface-emission laser module including a surface-emission laserdiode, said surface-emission laser diode comprising: an active layer; ann-type semiconductor distributed Bragg reflector; and a p-typesemiconductor distributed Bragg reflector, said n-type semiconductordistributed Bragg reflector and said p-type semiconductor distributedBragg reflector being disposed at both sides of said active layer, saidn-type semiconductor distributed Bragg reflector having an increasedresistance with respect to a region forming a cavity of said thesurface-emission laser diode.
 120. An optical interconnection systemincluding a surface-emission laser diode, said surface-emission laserdiode having an n-type semiconductor distributed Bragg reflector, saidaid n-type semiconductor distributed Bragg reflector comprising: firstand second semiconductor layers of n-type stacked with each other, saidfirst and second semiconductor layers having respective refractiveindices different from each other; and an intermediate layer providedbetween said first and second semiconductor layers, said intermediatelayer having a refractive index intermediate of said refractive indicesof said first and second semiconductor layers.
 121. An opticalinterconnection system including a surface-emission laser diode, saidsurface-emission laser diode, comprising: an active layer; an n-typesemiconductor distributed Bragg reflector; and a p-type semiconductordistributed Bragg reflector, said n-type semiconductor distributed Braggreflector and said p-type semiconductor distributed Bragg reflectorbeing disposed at both sides of said active layer, wherein said n-typesemiconductor distributed Bragg reflector is processed to form a mesa.122. An optical interconnection system including a surface-emissionlaser diode, said surface-emission laser diode comprising: an activelayer; an n-type semiconductor distributed Bragg reflector; and a p-typesemiconductor distributed Bragg reflector, said n-type semiconductordistributed Bragg reflector and said p-type semiconductor distributedBragg reflector being disposed at both sides of said active layer, saidn-type semiconductor distributed Bragg reflector having an increasedresistance with respect to a region forming a cavity of said thesurface-emission laser diode.
 123. An optical telecommunication systemincluding a surface-emission laser diode, said surface-emission laserdiode having an n-type semiconductor distributed Bragg reflector, saidn-type semiconductor distributed Bragg reflector comprising: first andsecond semiconductor layers of n-type stacked with each other, saidfirst and second semiconductor layers having respective refractiveindices different from each other; and an intermediate layer providedbetween said first and second semiconductor layers, said intermediatelayer having a refractive index intermediate of said refractive indicesof said first and second semiconductor layers.
 124. An opticaltelecommunication system comprising a surface-emission laser diode, saidsurface-emission laser diode, comprising: an active layer; an n-typesemiconductor distributed Bragg reflector; and a p-type semiconductordistributed Bragg reflector, said n-type semiconductor distributed Braggreflector and said p-type semiconductor distributed Bragg reflectorbeing disposed at both sides of said active layer, wherein said n-typesemiconductor distributed Bragg reflector is processed to form a mesa.125. An optical telecommunication system including a surface-emissionlaser diode, said surface-emission laser diode comprising: an activelayer; an n-type semiconductor distributed Bragg reflector; and a p-typesemiconductor distributed Bragg reflector, said n-type semiconductordistributed Bragg reflector and said p-type semiconductor distributedBragg reflector being disposed at both sides of said active layer, saidn-type semiconductor distributed Bragg reflector having an increasedresistance with respect to a region forming a cavity of said thesurface-emission laser diode.